Abstract: In order to study the effects of spacer grids on 5×5 rod bundles channel, laser induced fluorescence (LIF) technique was applied. The transverse exchange of the spacer grid on the flow field was obtained indirectly by the internal concentration distribution of the dye tracer (RhB) under normal temperature and pressure. The blending diffusion process of the dye tracer can be intuitively observed. The self-verification method was applied in order to check the accuracy of LIF technique measurement. By reconstructing the concentration of the dye tracer at different locations in the radial and axial positions of the rod bundle channel and comparing experimental results with spacer grid or not, the range of the influence of the spacer grid on the flow field was observed. The coefficient of variation (COV) was also applied to quantify the mixing intensity of the spacer grid. The results show that spacer grid can mix the flow quickly and the difference of the mixing vanes is the main reason cause of the cross-mixing strength and anisotropy. Under the Reynolds number is 10 478, the effect on the down-steam flow field is about 8 equivalent diameters (D_h) and the strongest mixing ability is at 5D_h downstream of the spacer grid.