The effect of flow structure on the particles traort and particles deposition pattern in a rigid, smooth-walled model of the human lung airways extending from trachea to the segmental bronchi is studied. In this work, based on the Horsfield morphometrical data of human lung, an out-of-plane model of three generation of human lung is produced. Particle deposition sites and efficiency studied for particles in the Stokes number range of, 0.02StkTrachea ? 0.102, at iiratory flow rates of 30 lit/min and 60 lit/min, measured at trachea. A commercially available CFD cod is used for numerical simulation. A lagrangian approach is employed, and one-way coupling is used between the continuum and dispersed phase, which allows particle tracking to be run as a post-processing calculation. The 3-D steady laminar flow is numerically simulated and then the particles trajectories are determined by numerically solving the Newton law. It is shown that the particle size has substantial influence on deposition regarding both efficiency and location. Due to asymmetry of model and high velocity at the first generation of human lung, effective mechanism at particle deposition is impaction. Therefore, Stokes number can show the efficiency of impaction mechanism. The result shows that with Stokes number increasing the deposition of particles increase. Gravity and Saffman force are effective at high Stokes number and Brownian force due to high air velocity at the airways has not any affect on particle deposition. Also it is noticed that for micron size particles deposition mainly occurs by inertial impaction at upper airways. Deposition efficiency with parabolic velocity at the inlet is about 50% more than realistic inlet condition. The results show that particle deposition in Weibel model is less than Horsfield model. Also had been shown more fraction of particles enter the lower lobes of human lung.