Abstract: The temperature fields in planar fuel assemblies are simulated numerically by Computational Fluid Dynamics(CFD) program CFX5. The results are useful in optimizing the design of a planar fuel assembly. The results indicate the velocity and temperature in different flow channels of a planar fuel assembly are relatively uniform, and the velocity and temperature in the same flow channel are also uniform. The variation of the flow velocity between different flow channels from outside to inside is small, and the flow velocity in the middle flow channel is relatively smaller than in other channels. The velocity becomes low when the flow is approaching the wall boundary, and the velocity at the wall boundary is zero. The temperature of the fuel plates and pectinate localizers is high, and the temperature of the side panels is close to the temperature of the flows between fuel plates, and the temperature of the flow outside of the side panels is low.