Experimental and theoretical studies were Bade of the separation efficiency of an air cyclone separator. The results of numerical calculation of three-dimensional Navier-Stokes equations indicate that the flow field chanqes with circumferential angle. The velocity vectors just below the exit tube are directed to the central axis at one circumferential angle, but the velocity vectors are directed to the opposite direction at other circumferential angle. The downward velocity components near the conical wall are large and the downflow and central upflow merge strongly near the entrance of dust box. It is also found that there are upward flow velocity components even in the upper part of the dust box. From the trajectory calculations of each particle, large particle are collected on the upper wall but small particles on the conical wall. It is also confirmed that particle with small inertia enter into the dust box first, but finally exit from the cyclone because of the upward velocity component. The experimental partial separation efficiency obtained agree well with the numerical calculation when the data are rearranged according to the particle inertia parameter.