Abstract: Mixed exponentially distribution failure time distributions play an important role in life time data analysis. But we may encounter enormous difficulties when we try to estimate the model by traditional methods such as maximize likelihood (ML) estimation and moment estimation. In this paper, we employ the EM algorithm to obtain the ML estimation for the mixed exponential model under the normal stress life time test with full data or censored samples. The iteration formula are given in the paper. Simulations are also made to evidence the performance of estimation.