Abstract: In nuclear neutronics/thermal-hydraulics coupling analysis, grid size will affect the speed and accuracy of calculation. This paper assesses the effect of the grid size on coupling results by establishing a PWR cell model at different levels of grid size and coupling calculation using the Monte Carlo neutron photon transport code MCNP5, the computation fluid dynamics code FLUENT, and our interface program MF_COUP. The relationship of grid temperature difference, grid density difference, and relative errors of the infinite neutron multiplier and power density distribution are demonstrated. The grid size causes errors less than 10-4 to keff and errors less than 1% to power distribution while the gird temperature difference of fuel is less than 50 K and the grid density difference of moderator is less than 3 kg/m3. Furthermore, a PWR single cell model and a PWR 33 cell model are calculated based on our mesh scheme. The results show that the using our mesh scheme is concise and effective while the total number of grid and computation time decrease significantly.