ABSTRAK This study aims to determine the effect of market reaction can be measured by using abnormal return on stocks incorporated in the Jakarta Islamic Index before and after the events of the company reentry at the Jakarta Islamic Index. Abnormal return is derived from the difference between realization return and expectation return. The type of investigation of this research is event study. This research is a quantitative research in which data collection is done by documentation method. Population used in this research is company registered in Jakarta Islamic Index (JII) period 2011-2015. The sample selection is done by purposive sampling method with the aim to get the sample which is suitable with the purpose of the research. While the process of analysis is done by basing on the method of quantitative descriptive analysis. The results showed that the results of the test through the Paired Sample t-test showed the results of sig. (2-tailed)> alpha (0.001 alpha (0.001