A gas flow through a turbomolecular pump is modeled by the direct-simulation Monte Carlo method over a wide range of gas rarefaction covering the free-molecular, transitional, and hydrodynamic regimes. Several values of the rotor speed are considered. The main characteristics of the pump, such as maximum pumping speed and compression ratio, are reported. It is pointed out that in the range between the free-molecular and transitional regimes, the pump characteristics change slightly, but they vary significantly in the hydrodynamic regime. It is shown that the pumping speed increases in the hydrodynamic regime when the rotor speed is high, but it decreases for a small value of the speed. The compression ratio always decreases when approaching the hydrodynamic regime. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3484139]