This study aims to investigate the effects on stock prices of added or removed component stocks in Taiwan 50 Index and Taiwan Mid-Cap 100 Index. Event Study Method is used in analyzing the average (cumulative) abnormal returns of added or removed component stocks in the two indexes before and after the declaration date and effective date. The empirical results show that the added and removed component stocks of Taiwan 50 Index and Taiwan Mid-Cap 100 Index respectively create positive abnormal returns and negative abnormal returns at the event declaration date and effective date.