A cyclone dust collector is a simple device with no moving parts,so it is applied in many industrial fields. However there is still no satisfied theory to estimate the collection efficiency for a given cyclone with high reliability since it is very difficult to predict the movement of the fine solid particles in the three dimensional turbulent rotational flows in the cyclone. A new model of the fractional collection efficiency including the diffusion effect of the solid particles and the decay effect of the tangential velocity of gas flow along the concave wall sursace has been derived. In order to confirm this model, a simplified axial flow cyclone of body diameter D-1=99mm has been constructed and fly-ash particles were used as a test dust. The experimental results of collection efficiency were compared those predicted by the Ogawa model and the Fuchs model.