Abstract: The mixing of cooling fluid between two adjacent sub-channels in rod bundles reduces significantly the wall temperature difference along the perimeter of the rods. In order to study the special structure of flow in tight lattice, experiment investigation and numerical simulation were performed in a 2×3 square channel with P/D=1.1. Dye tracers were adopted in the experiment to visualize the detailed flow structure in the narrow gap between rod and wall. The Re was varied from 2 000 to 40 000. Large scale quasi-periodic oscillation was observed. Some characteristic parameters of oscillation were obtained. The results show that the large scale oscillation occurs at Re ≥5 000. The wavelength of oscillation is independent of Re, whereas the oscillation frequency increases proportionally with Re. Large scale quasi-periodic oscillation in the same cross section is also obtained using CFD simulation with SSG turbulent model. The calculated characteristic parameters agree well with the measured data.