Abstract: The experiment investigation on the turbulent flow in a 6×6 rod bundles was conducted with three-dimensional laser Doppler velocimetry （LDV）. Five cases were selected to carry out the experiments at the Re ranging from 6.6×10³ to 7.03×10⁴, in which the geometry of the 6×6 rod bundles was used to simulate the layout of two adjacent fuel assemblies. There are two LDV measuring positions in the experiment. One is the side view of the experiment facility where the axial velocity and turbulence intensity at different distances from grid spacer were obtained. The other measuring position is at outlet of the rod bundle where three-dimensional velocity distribution and Reynolds stress distribution were measured. By comparing the results at different Re, the influence of Re on the experiment was analyzed. The low Reynolds number effect is found with Re=6.6×10³.