Particles deposited on the surfaces of ducts could be a second source of in door air pollution. From a previous field study, it was found that 78% of occupants didn't use mechanical ventilation fans in Korea, and the occupants frequently opened windows to ventilated indoor air, A model was estimated the deposition in ducts was developed through theoretical analysis, and the model was estimated with the measured data in 14 sample apartments. the deposition amount of particles were calculated using the particle resuspension equations. the results of research showed that the amount of particles deposited on the supply air ducs was significantly influenced by the built years and the mechanical fan operation.