Abstract: One of the most important components in PWR fuel assembly is spacer grid and mixing vane attached to grid strap has a significant influence on mixing effect in rod bundles. The analysis of cross-flow mixing between rod bundles is of great importance to innovative design of fuel assembly, especially for bundles with mixing vane grids (MVGs). Using particle image velocimetry (PIV) technology and MIR method, this paper obtained cross flow in the lateral planes downstream two 5×5 rod bundles. There were two types of MVGs in the experiment, namely MVG Ⅰ and MVG Ⅱ. The MVGs were installed on the bundles respectively with MVG Ⅰ on the left and MVG Ⅱ on the right. The geometrical parameters of these two types of MVGs were different but both the mixing vanes were split-type with the bending angle of 29°. Besides, the ratio of strap height of two grids was 1.744 and the ratio of surface area of mixing vanes was 1.486. Considering the non-uniformity of lateral flow field between rod bundles caused by height difference of grids, two kinds of arrangements of MVGs were chosen: 1) the axial centers of two straps were in alignment; 2) the axial center of MVG Ⅰ strap was aligned with the top of MVG Ⅱ. The cross flow in the inter-assembly area was measured in the lateral planes of 1.8D_h-25D_h downstream MVGs. The measurement was conducted under condition of room temperature and atmospheric pressure. To quantify the mixing effects induced by MVGs, cross-flow intensity in the gap between inter-assembly area and its adjacent subchannels and secondary-flow intensity in the inter-assembly subchannels were calculated. The results showed consistency in cross-flow trend at different Reynolds numbers, thus the experimental data at a Reynolds number of 13 200 were used for analysis. The experimental results indicated that the mixing vane pairs in the inter-assembly subchannel induced vortices which turned to inter-subchannel cross flow and enhanced the lateral mixing between bundles. The results also showed that the secondary-flow intensity within the inter-assembly subchannel and cross-flow mixing between inter-assembly area and the adjacent subchannels were both effected by axial location difference of MVGs and the arrangement of mixing vanes. The larger the axial height difference of the MVGs, the more uneven the cross flow mixing between the inter-assembly area and the adjacent subchannels. In general, the local cross-flow mixing within the inter-assembly subchannels and cross-flow intensity between assemblies downstream the B-type arrangement of MVGs were stronger than A-type.