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In-situ stirring batch reactor transesterification is a widely used method for the conversion of various biomass into biodiesel, particularly Nannochloropsis sp microalgae. Therefore, this study aims to investigate the effect of CaO/Hydrotalcite catalyst loading ratio and stirring rate on in situ stirring batch reactor transesterification of Nannochloropsis sp microalgae to produce biodiesel. In-situ transesterification was carried out using stirring with a four-blade paddle (50 w), while CaO/Hydrotalcite catalyst preparation was performed using incipient wetness impregnation. Nannochloropsis sp microalgae were then converted into biodiesel with varying loading ratio of catalyst (1:1, 3:1, and 5:1 %wt/wt) and stirring speed of agitation (50, 100,150, 200, and 250 rpm) for 4 hours (60oC). Subsequently, biodiesel obtained from the process was analyzed using gas chromatography (GC). The results showed that the surface area of CaO/Hydrotalcite was 45.756 m2/g. The conversion yields of Nannochloropsis sp microalgae to biodiesel with CaO/Hydrotalcite reached 38.36% with an agitation rate of 250 rpm. Based on the fatty acids analyzed using GC methods, the products predominantly contained olefine (61.49%) and gasoline (38.51%). [ABSTRACT FROM AUTHOR]