While the concept of corporate governance is a concept that continues its existence for a long time with different applications, even if not with its present meaning, especially after Enron, which took place in 2001 in the United States, and Parmalat scandals that emerged in Italy in 2003, it was a phenomenon that attracted the attention of all countries not only those countries. Corporate governance is a philosophy of management in itself, a philosophy that holds regulations on all aspects of businesses and aims to manage the relationship of all these aspects to each other based on a solid ground. To do this, the principles laid down by the OECD are regulated under such headings as i) shareholder rights and fundamental property functions, ii) equitable treatment of shareholders, iii) roles of stakeholders, iv) public disclosure and transparency, and v) responsibilities of board of directors. On the light of these information, The BIST Corporate Governance Index is calculated on the basis of the price and performance of companies whose shares are traded on the Stock Exchange Istanbul markets (except for the Close Monitoring Market and C and D lists) and which have at least 7 out of 10 compliance scores and at least 6.5 out of 10 aimed to measure return performance Methods In this study, there are 50 companies listed on the Borsa Istanbul Corporate Governance Index, which have issued corporate governance rating notation announcements in 2015. The study aims to determine how investors evaluate the announcement of the corporate governance index by examining the movement in stocks. In this direction, the event study method will be utilized. In the literature the event study aims to reveal the effect of the event by evaluating the values before and after the date or dates determined with a number of statistical methods. After calculating the real rate of return for 50 companies and the market rates of return for 50 companies, the abnormal returns will be calculated for each day from 15 days before the announcements to 15 after the announcements. Cumulative abnormal return rates for each event window will then be calculated and compared. At the end of these calculations, cumulative abnormal return rates before the announcement date will be compared with cumulative abnormal return rates after the announcement date. On the other hand, the reactions of company stocks to the announcements for 2015 were evaluated and interpreted in three different event windows. Findings It is seen that the average of 13 days from the average abnormal returns obtained have negative returns. Six of them were realized after the announcements and seven before the announcements. This can be implied that stocks are not always come up with positive reactions after the province and that there may also be value decreases due to profit taking or other reasons. Another point to note is that the highest ratio within the mean abnormal returns is the day after the announcement day (t + 1 = 0.88). On the day of the announcement, it has the fourth highest average. It is understandable that when the notice is taken at the time of the announcement, the day of the announcement gives less abnormal returns compared to the next day. On the third day following the announcement, the abnormal return was negative. The average abnormal return on the day of the announcement and the following two days is calculated as 1.70%. When the 31-day event window is examined, the cumulative abnormal return rate from the announcement date to the day before the announcement is 2.40%, while at the end of the 15th day the rate is up to 4.56%. A similar situation can be seen from the 21-day event window. Cumulative abnormal return up to the day before the announcement day is 1.03%, but it rises to 3.09% at the end of the 10th day. Lastly, when taking into account the possibility that the announcement effect is much shorter, the cumulative abnormal return rate from the announcement date to the day before the announcement was calculated as 0.29% for the 11-day window, at the end of the 5th day this rate was 2.11%. In all three event windows, there are positive increments between the day before the announcement and the day at the end of the window. From this point, similar results are obtained in the new event window of 15 days before the event and 2 days after the event. Cumulative abnormal return before the announcement was 2.70%, but at the end of the second day, this ratio increased to 4.09%. Another noteworthy situation is that the third trading day after the announcement is below the market return. This situation is likely to be regarded as a result of profit taking (profit realization) of stock market users for speculative purposes. Conclusion In this study, cumulative abnormal return ratios after the announcement obtained on a daily basis in 3 different event windows has higher return than before announcement. After the announcement of the rating note, stock returns increased, so the market value of the companies also increased. The semi-strong form, the second degree of the Efficient Market Hypothesis, assumes that investors can not make abnormal returns by exploiting the companies public statements in addition to historical data. (Karan, 2004). The results obtained in the study contradict the Effective Markets Hypothesis. Hence, the result is that the BIST is not even effective in the semi-strong form. This study was based on the announcement of the rating marks of the old and new companies that were included in the same year. Similarly, a study from a previous year can be evaluated as a research topic and provided with the possibility of comparison. It will be possible to diversify and improve this work by making use of the differences in abnormal return calculation methods. [ABSTRACT FROM AUTHOR]