According to Kalām thought, existence is divided into qadīm (eternal) and hādith (contingent being). The opposition between God, who is the qadīm, and the universe, which is created, is essential for proving the qadīm's existence and for negating from Him the characteristics of the hādith. Due to this background, the atomist theory is critical for understanding the Kalām tradition. It was the Mutazilites who incorporated the atomist theory into Kalām and harmonized it with Kalāmic thought. In this context, it should be noted that atomism developed in Mu'tazilite Kalām through criticisms against atomism and intra-sectarian disputes. The Ash'arite mutakallimūn, in opposition to the Mu'tazilites on many issues, followed the Mu'tazilites about the conception of the universe and adopted the atomic theory. Ashʿarite mutakallimūn did not ignore the transformation of atomism in Mu'tazilite Kalām but they reinterpreted the theory according to their Kalām systems. Since previous studies on atomism have mainly focused on the Mu'tazilites, this study focuses on the Ash'arites' understanding of substance. The reason for putting the substance at the center of the study is that it is more critical for atomic theory than accidents. Another reason for choosing substance as the subject is that accidents have already been the subject of other studies. This study differs from the present literature not only in terms of its choice of topic but also in terms of the sources it uses and the names it deals with. This study aims to identify and classify the issues related to substance in an original manner. It begins by discussing the definition of substance in Ash'arite Kalām, followed by examining the views of Ash'arite mutakallimūn on features of substance such as taḥayyuz and tamakkun, indivisibility, tejānus (homogeneity), direction, shape, state at the moment of hudūth, baqā (continuance in existence), fanā (ceasing to exist), and return. The study also analyzes opinions about the relationship between substances and accidents and the composition of substances within the Ash'arite sect. Additionally, the study includes the views of Ash'arites on the void. The study includes notable Ash'arite mutakallimūn from the mutaqaddimīn period, such as Abū al-Hasan al-Ash'arī, Abū Bakr al-Bāqillānī, Ibn Fūrak, Ishaq al-Isfarāʾīnī, al-Bāqillānī's student Abū Ja'far al-Simnānī, al-Baghdādī, Imām al- Haramayn al-Juwaynī, and his contemporary al-Mutawallī. These perspectives of these names on substance form the basis of the discussion, allowing for tracing of atomism's course in Ash'arite Kalām and identifying of each mutakallim's role in its development. In conclusion, this study of the understanding of substance in the Ash'arite Kalām of the Mutaqaddimīn Period reveals that, with some exceptions, Ash'arite mutakallimūn held similar views on the subject. However, this does not imply that atomism did not develop in Ash'arite Kalām. On the contrary, the development of atomism among Ash'arite mutakallimūn took the form of elaboration of views, diversification of arguments, and strengthening of weaknesses. As a result, it has become clear that the views on atomism and substance in the early Ash'arite Kalām constitute an excellent example for determining the general structure of the sect, the intellectual relations with the opponents, and the role of the mutakallimūn in the development and history of the sect. [ABSTRACT FROM AUTHOR]