The distribution of the five-day's return on China composite stock index is studied. The empirical distribution of five day's return on SHCSPI(ShangHai composite stock price index) and SZCSPI(ShengZhen composite stock price index) has "fat tail" and no finite variance with sharp peak at mean ,which can not be normally distributed because the largest negative return possible. In addition, the return series are belong to long-turn memory fractional time series. China stock markets are not efficient markets because the chaos phenomenon is strongly shown in the stock market.