Abstract: Spacer grid is one of important components in fuel assembly. Not only do spacer grids provide rods with supporting and positioning, but also their thermal hydraulic performance is significant in fuel assembly. The cross-mixing of spacer grid for the working fluid is closely related to the economy and safety of reactor. Therefore, an investigation on mixing characteristic of spacer grids in fuel assembly is indispensable. A visualization research on flow field upstream and downstream the spacer grid was implemented by the particle image velocimetry (PIV) technique. In order to obtain a global understanding of mixing characteristic of spacer grids, comparisons of the velocity distribution in a rod bundle with and without spacer grids were performed, and the mixing contribution of spacer grids for flow field was quantitatively evaluated. The lateral velocity and axial velocity downstream the spacer grid for the different inlet boundary conditions were obtained, and mixing characteristics of spacer grids for lateral velocity and axial velocity were analyzed under different flows. In addition, the turbulence characteristics downstream the spacer grid were assessed. Experimental results benefit the development and evaluation of spacer grid, and also provide the whole-field validation for numerical simulations.