The computational fluid dynamics (CFD) technique is used to simulate airflow and aerosol-particle deposition in human respiratory tract. The three dimensional respiratory tract is extended from third to six generation bronchus based on Weibel model (1963). The computation is performed for laminar flow condition. Discrete phase modeling (DPM) is used to study the two phase flow. The aerosol-particles of three different diameters 1, 5 and 10 μm are considered to obtain the effect of aerosol-particle size on deposition efficiency. The results of velocity contours, aerosol-particle deposition efficiency and aerosol-particle trapping process are determined at different locations of the respiratory tract. It is obtained that deposition efficiency increases with the increase of aerosol-particle size. It is also found that more are deposited at the bifurcation junctions because of the direct inertial impaction at these locations. Computational results are found useful to correlate the medical and engineering aspects.