Abstract: In order to investigate the influence of the spacer grids and mixing vanes on boiling crisis in nuclear reactor, the effects of number, location of spacer grid and bending angle of mixing vane on the flow and heat transfer in rod bundle are studied by using the computational fluid dynamics (CFD) analysis method. The results show that the spacer grid will cause resistance to the mainstream flow, and the peak temperature value is increased along with the number of spacer grid. However, the peak value of temperature can be restrained to a certain degree when the spacer grid is arranged in the position where boiling crisis occurs. . The mixing vane can decrease the void fraction in near wall region to improve the heat transfer effectively. But the mixing vane with too large bending angle will bring the boiling crisis in advance.