The stock market of the world often relate to each others. Especially, USA, the economic powerful country, is usually the leader. This research discusses that the Philadelphia semiconductor index (PSI) of U.S.A impact on the stock price of Taiwan IC industry. We use VAR model to explore the lead-lag relationship of the stock return between the PSI and the stock price of Taiwan IC industry. The research period is from 2005/7/4 to 2006/6/30. The results show that the stock price of Taiwan IC industry are strongly influenced by PSI and the impact of stock price of Taiwan IC industry from PSI reflects within 2 cycles.