The purpose of this study is to investigate whether intraday overreaction occurs in Taiwan index futures market using the standard event-study methodology in Fung, Mok, and Lam (2000) and Grant, Wolf, and Yu (2005). Our empirical results show no evidence of price reversals. On the contrary, this study finds significant intraday momentum effects following large price changes at the market open, suggesting that there exists overnight underreaction phenomenon in Taiwan index futures market. The results will be helpful for the authorities that regulate futures market to improve market efficiency.