This article presents a numerical study of indoor airflows and contaminant particle transportation in three ventilated rooms. The realizable k - epsilon model is employed to model the air-phase turbulence, while the Lagrangian particle tracking model is utilized for the particle-phase simulation. The predicted air-phase velocities and contaminant particle concentrations are validated against the experimental data obtained from the literature. In the first case, the realizable k - epsilon model successfully captures the flow trend and reasonably predicts the airflow velocity. The realizable k - epsilon model under-predicts the vertical air velocities along the vertical inlet jet axis by 11% at x = 0.219 m, which is slightly better than the standard k - epsilon model error of 17%. In a two-zone room case, the realizable k - epsilon model, combined with a Lagrangian particle tracking model, predicts the particle concentration decay with the highest normalized difference being 24%. In the third case, the influence of particle size, location of particle resource, and particle-wall collision on the particle concentrations is investigated by the realizable k - epsilon model and the Lagrangian model. It is found that for relatively small particles ( diameter <= 10 mu m), the particle concentration may be insensitive to the particle diameter. In addition it has been observed that the particle-collision model may have considerable effect on the particle concentration prediction.