In this paper the MBE nucleation mode on vicinal GaAs(001)surface is simu-lated by Monte Carlo method. The results show that on A-surface the 2-dimension nucleation mode is dominant at 800-1000K. On B-surface the 2-dimension nucleation mode is dominant at low temperatures, but at high temperatures the step flow mode is dominant. And at both high and low temperature, there is a saturation phenomenon.