The pumping characteristics of a single-stage turbomolecular pump from free molecule region to transition region are calculated by the DSMC method. The formulation is fully three-dimensional and derived on the rotating frame. Variable soft sphere (VSS) model and No Time Counter (NTC) collision scheme are used. The intermolecular collisions are regarded as inelastic by applying the Larsen-Borgnakke model. Numerical results show good quantitative agreement with existing experiments in the free molecular region, and have greater discrepancy as the Knudsen number decreases.