Abstract: The turbulent flow in 37 rod bundles was investigated with CFD code Fluent. The calculation results were validated with experimental data. The effects of diminishing rod-rod gap size on local flow, heat transfer and coherent structure in tight lattice were analyzed. The critical pitch to diameter ratio is next to 1.03. As the decreasing of pitch to diameter ratio, the coherent structure and flow mixing increase first and then decay sharply. In the very narrow gap, the coherent structure is so weak that it can be omitted. The oscillations of velocity, wall shear stress and wall temperature are also very weak. But the spatial difference of the parameters in this gap is very significant.