Abstract: The single-phase sub-channel mixing mechanisms in rod bundle include diversion cross-flow mixing and turbulent mixing. The split-type mixing-vaned spacer grid strongly enhances cross-flow mixing and turbulent mixing, resulting in lower peak temperature on rod-bundle surface and higher critical heat flux. Thus, the split-type mixing-vaned spacer grid is an important device in fuel assembly for pressurized water reactor (PWR). However, the enhancement mechanism of spacer grid is still not clear. And clear enhancement mechanism of spacer grid mixing effect on sub-channel mixing is the basement for mechanical sub-channel mixing models. To study the spacer grid mixing effect, the cross-flow and longitudinal flow downstream a split-type mixing-vaned spacer grid in a 5×5 rod bundle was measured by particle image velocimetry (PIV), with sub-channel Reynolds number (Re) from 6 600 to 39 600. The rod bundle was fabricated with diameter 9.5 mm, pitch 12.6 mm and sub-channel hydraulic diameter 11.78 mm. The maximum standard uncertainty of statistical quantities of turbulence normalized by the bulk velocity is less than 1%. The secondary flow structure downstream of the spacer grid is dominated by the vortex shedding from mixing vanes near spacer grid within 3Dh, and then the cross-flow develops and builds up after the dissipation of shedding vortices. The strong shear flow generates co-rotating vortices in each sub-channel. With flow development downstream of the spacer grid, the two vortices expand, weaken and become close to each other, and they merge into single vortex covering the whole sub-channel. Then the single vortex decays slowly. The vortex structure pattern coversion impacts on the lateral mean velocity and root mean square fluctuating velocity. Thus, the vortex pattern determines the sub-channel mixing, including the diversion cross-flow mixing and turbulent mixing. Near the space grid, lateral turbulent intensity is strong in the shedding vortices and decays fast with the dissipation of shedding vortices. With double-vortices generation by strong shear-flow, the lateral turbulent intensity is strong in the vortices again and decays fast with the dissipation of double vortices. Far from the spacer grid, the lateral turbulent intensity decays slowly because the cross-flow decreases slowly. The turbulent flow in rod bundle tends to fully developed flow in bared rod bundle far from spacer grid till z=28Dh. Both the mixing effect of spacer grid and Reynolds number has important effect on the cross-flow. Near the spacer grid, mixing effect of space grid manipulates the cross-flow features, while the Reynolds number effect controls the cross-flow far from the spacer grid.