Abstract: Turbulent flows in a rod bundle are important field for the nuclear reactor fuel assembly since the flow mixing in a sub-channel or between sub-channels can significantly modify the heat transfer performance. A five beam three dimension Laser Doppler Velocimetry（LDV）system is applied to measure the flow velocities in a 3×3 rod bundle installed with the simple grid spacers and without mixing vanes. The rod diameter and spacing between rods are set according to the typical fuel assembly. The area to be measured is near the outlet of the bundle, and it is far from the space grid, so it can be treated as the developed flow condition. 2D LDV is used to measure one quart of the flow cross-section, and the main velocity and main turbulence intensity have been recorded.