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Abstract: This paper adopts computational fluid dynamics (CFD) to numerically analyze particle deposition in the ventilation duct. A three-dimensional drift-flux model combined with particle deposition boundary conditions for wall surfaces is presented. The numerical method is used to analyze the particle deposition velocity and deposited particle mass flux in the ventilation duct after validation. Twelve groups of particle size, two average air speeds in ducts are investigated to understand the particle deposition in the straight ventilation duct, which ensures a fully developed turbulent duct flow. And then, the particle accumulation by deposition in the ventilation duct is analyzed according to the cleaning code for air duct system in heating, ventilation and air conditioning (HVAC) systems of China. The cases with or without air filter installed are studied by assuming that the duct inlet particle concentration is that of outdoor air in Beijing city, China. The simulated results of dimensionless deposition velocity onto floor agree well with the measured data from others, while the discrepancies of vertical wall and ceiling are obvious. Both the simulated results in this paper and measured data from literature show that particle deposition onto the floor (upward wall) in the ventilation ducts is the most significant. The deposition velocity onto the floor is about 2 orders of magnitude larger than that onto the other walls of the duct. The filter is effective to defend the ventilation duct against particle pollution by deposition. A higher efficiency filter is helpful to postpone the cleaning time for ventilation duct, so the filter should be replaced often to maintain the filter efficiency. [Copyright &y& Elsevier]