In the capital market, each stock price must reflect all available and relevant information or commonly known as the efficient market. In an efficient market, market anomalies should not occur to affect abnormal returns. This study aims to test whether the Indonesia Stock Exchange has January effect and Size Effect anomalies that can affect the capital market to be inefficient. The population of this study were all banking stocks listed on the IDX in the 2018–2022 periods. From this population, incomplete historical data samples will be excluded from the calculation. Therefore, the sample that meets the criteria is 38 banks. The research is divided into two parts: the first to examine the January effect and the second to exam- ine the Size Effect. In the first study, researchers tested whether there was a significant difference between abnormal returns in January and other months. Statistical tests use the Wilcoxon Signed Rank Test. In the second study, researchers tested whether there was a significant difference between the Average Cumulative Abnormal Return (ACAR) of large market capi- talization stocks and the Average Cumulative Abnormal Return (ACAR) of medium and small market capitalization stocks. Statistical tests use the Paired Samples T-test. The conclusion of this study is that there is no effect of the January effect for the 2018–2022 period except for 2021 period and no effect of the Size Effect anomalies on banking stocks listed on the IDX for the 2018–2022 period.