Abstract: The research activities about tight lattice rod bundles were ongoing worldwide to optimize nuclear rod bundles. However, there still exists large deficiency in research of the effects of spacer grid structure. In this paper, the heat transfer experiment in a tight-lattice 7-rod bundle with spacer grids was simulated with the computational fluid dynamics (CFD) code. The numerical simulation was validated by comparing numerical results with experimental data. The extra simulation was conducted for the bare rod bundle to investigate the effects of grid structure on the flow and heat transfer. The investigation results show that the maximum surface temperature and the derived average turbulent mixing coefficient are sensitive to the structure of grid.