We calculated fan performance and aerodynamic noise in the centrifugal fan of air conditioner by large eddy simulation (LES). In this study, we investigated simulation technology employing tetra-prism grids for practical usefulness. Tetraprism grids are easier to generate than hexahedral grids. We employed the numerical simulation code FrontFlow/blue (FFB) throughout the LES. First, we proposed a design method for tetra-prism grids. The design method featured a predicted boundary layer thickness that was the same as the thickness of a prism layer. Next, we compared calculated results for the 13 million grids, 107 million grids and 860 million grids to investigate the grid number influence on fan performance and aerodynamic noise. We confirmed that calculated results for larger number of grids was more accurate than smaller number of grids. We also confirmed that calculated results simulated streaks well and the number of streaks increased in the order of the increasing number of the grids. The proper simulating of the streaks therefore contributed to getting better calculated results. As a result, we confirmed that using the tetra-prism grids was practical in the actual development of fans.