Abstract: Based on the validation of CFD models with Hollway experiment results, two groups of CFD models about 5×5 rod bundle with separate or split mixing vane grids were respectively setup and simulated. The field synergy principle was used to discuss the mechanism of heat transfer enhancement by mixing vane according to the CFD results. The study shows that the field synergy principle is feasible to explain the mechanism of heat transfer enhancement in fuel assembly and the heat transfer enhancement can be predicted by the synergy angle distribution. If the pressure loss is ignored, the performance of the split mixing vane is superior to the separate mixing vane based on the enhanced heat transfer. Increasing the blending angle of separate mixing vane does not significantly enhance the heat transfer in the rod bundle, and even prevent the heat transfer at a large blending angle.