Abstract: In order to find out different effects for different parts of spacer grids, 3-D single-phase flow field in 5×5 rod bundles with spacer grids by numerical method was studied. By using hybrid grids technique, SST k-ω model and SIMPLEC algorithm, the Reynolds averaged mass conservation equation and momentum conservation equations were solved, and the pressure and velocity fields were obtained. The results show that the mixing vane is the main factor of lateral velocity, spring and dimple influent the axial velocity, drag coefficient largely, and the effect of spring is larger. The mixing vane has the effect of blending and averaging the axial velocity.