Abstract: Based on the heat conductivity principles, a mathematic model about pulsed laser induced crystallization of a-Si thin film had been developed. With the finite difference method, the effect of varied parameters, such as wavelength and energy density of laser, on the temperature distribution and phase change of the thin film during crystallization was analyzed. The melt threshold energy densities for 500 nm a-Si irradiated with different lasers were calculated. The results show that the crystallization with excimer lasers requires the lowest threshold energy densities. However, it does not create the deepest melt depth, compared with green or red lasers. The effect of different substrate temperatures on the crystallization speed and grain size was analyzed, and the calculated results accord