Abstract: The Laser Doppler Velocimetry (LDV) system is applied to measure the axial flow velocity on four cross sections downstream the 5×5 rod bundle with two mixing vanes. Mean velocity and RMS velocity on each cross section is obtained and by comparing the mean velocity and RMS velocity on different cross section, the distribution pattern downstream the mixing vane grids is analyzed. By comparing the experiment data downstream the two mixing vane grids, the effect of upstream flow field on the grid effect is analyzed. All experiment data is acquired based on sufficient repetitive experiment, and the data can be used for the validation and assessment of CFD computation results.