Abstract: this paper researches the distribution status of Shanghai synthesis index returns and Shenzhen composition index returns by use of modified Weibull distribution.The results show that the modified Wibull distribution can completely depict the distributions of Shanghai synthesis index returns and Shenzhen composition index returns through the simple shift transformation;The big returns obeys sub-exponential distribution and the small returns obeys super-exponential distribution;There are a few differences between the probability distributions of two stock index returns.The fluctuatio of Shanghai synthesis index in bigger than that of Shenzhen composition index;After 1996,the probability distributions of stock market returns in Shanghai and Shenzhen have biggish change and the change of Shanghai stock index returns is more big.

Key words: stock market of China, returns, modified Weibull distribution