Particle deposition in a fully developed turbulent flow of ventilation duct with a rectangular cross section of 0.3 m×0.2 m and 3 m in length was investigated. The effects of thermophoretic force and air humidity were also considered. Trajectories of 3 000 particles were tracked on the basis of the random walk model from Lagrangian approach. The profile of dimensionless deposition velocity with relaxation time was achieved and the comparison with the previous studies was made with particle size range of 0.0150 μm and air flow velocity of 5 m/s. It is found that the trend of dimensionless deposition velocity is in agreement with the previous studies and the profile of dimensionless deposition velocity showed a Vshaped curve. The effects of temperature and air humidity were also investigated with particle size of 1 μm. Thermophoretic force caused by temperature gradient between air flow and wall surface accelerated particle deposition. Meanwhile, it is also found that the deposition velocity increases with the increase of air relative humidity.