This paper examines whether the trading behaviors of foreign institutional investors are different before and after US sub prime mortgages crises in July, 2007. The results show the positive feedback behaviors are more significant after sub prime mortgages crisis. Moreover, the foreign institutional investors buy (sell) lots of stocks, but only result in abnormal return of the stocks in short term. The behaviors will not affect the stability of the stock market. Further, the crucial finding is that the foreign institutional investors usually focus on the stocks of large companies and buy the winners and sell the losers before US sub prime mortgages crises. However, they focus on the stocks of medium and small companies during sub prime mortgages crises.