Abstract: A CFD method to evaluate two-phase mixing characteristics on PWR fuel rod bundle with spacer grids was presented in this paper. Numerical simulations on two-phase flow were carried out with CFX12.0 for two different types of spacer grid, and the simulating results were compared with the results of critical heat flux (CHF) test with Freon. The results show that the CFD method applied to predict bubble distribution characteristics in the downstream region of space grids is preliminarily validated.