The distribution of particles in a room is of great interest because of the effect of particles on human health. Using computational fluid dynamics, it is possible to study the behaviour of particles in a room. In this study, the commercial software STAR-CCM+ was used to simulate the dispersion of passive particles in a room with displacement ventilation system. In addition, some experiments are performed to verify the accuracy of the simulation results. According to the comparison of the experiment and the simulations in front of supply opening, the k–ε model seems to give better results than the k–ω model. It is shown that, in order to have an accurate result, the simulation of radiation effect is essential. Furthermore, the results of particle simulations show that when the passive particle source is in the height of 1.5 m and the heater is at the height of 0.5 m, the average concentration in the room is the lowest. Depending on the particle source height and heater location, the average particle concentration profile will change. In remote areas, the concentration profile does not show any significant difference between upper zone and lower zone.