Abstract: In atomic vapor laser isotope separation, the metal is heated and melted by electron beam, and some atoms are on the metastable states. In order to analyse the influence of metastable states' deexcitation on vapor physical parameters, such as velocity, temperature, direct simulation Monte Carlo method is used to simulate the 2 D gadolinium evaporation from long and narrow crucible. The simulation results show that the metastable states' deexcitation increases the velocity and temperature of the vapor, and the smaller Kn is, the more frequently atoms collide, the larger influence of metastable states' deexcitation is.