The direct simulation Monte Carlo (DSMC) method is applied to investigate steady and unsteady flow fields of a single-stage disk-type drag pump. Two different kinds of pumps are considered: the first one is a rotor-rotor combination, and the second one is a rotor-stator combination. Each of rotor and stator has 10 Archimedes' spiral blades, respectively. In the present DSMC method, the variable hard sphere molecular model with the no time counter technique is employed to simulate the molecular collision kinetics. For simulation of diatomic gas flows, the Borgnakke-Larsen phenomenological model is adopted to redistribute the translational and internal energies. Some experimental works are carried out to verify the numerical results. Good agreement between the numerical results and the experimental data is shown.