Banking is very regulated by the government and even has to follow regulations issued by the Basel Committee on Banking Supervision, which regulates banking in the world. According to Basel III, banks must provide capital reserves called capital buffers. The purpose of this study is to examine the factors that determine capital buffer. Factors thought to affect the capital buffer studied consisted of profitability (ROA), credit risk (NPL), liquidity risk (LDR), capital adequacy in the previous period (CARt-1), management risk (NIM), and ratio of operating risk (OER). The population in this study is conventional banks listed on the Indonesia Stock Exchange, as many as 42 banks, with a sample of 40 banks taken by purposive sampling method with an observation period of four years with quarterly data (2016-2019). To test the hypotheses, regression panel data is used. After being tested, it turns out that the fixed effect model is better than the common effect and random effect. The results of the study with fixed effect models show that ROA, NPL, and OER significantly and negatively affect capital buffer. CARt-1 has a positive and significant effect on capital buffer, while LDR and NIM do not affect capital buffer.