Turbomolecular pumps (TMPs) coated with SiO2 have relatively rough surfaces and give a higher compression ratio than non-coated pumps. In order to investigate the effect of the surface roughness on the pumping performance, the surface roughness was modeled by statistically located circular cone peaks and dimples of the same base radius and various base angles. The actual surface roughness of a blade was measured by SEM, and the obtained angle distributions were used to the surface roughness model. The effects of the surface roughness on the pumping performance of TMP were estimated by a two dimensional and a three dimensional Monte Carlo simulations. The calculated results were compared with the previously measured maximum compression ratios and pumping speed factors. The predicted results showed almost the same values as the experimental ones.