The velocity boundary layer formed on the collecting wall surface of a cyclone plays an important role as a barrier for particle deposition due to a sharply decreasing centrifugal force near the wall. A new collection efficiency model for small cyclones is developed, where turbulent diffusion in the core flow region and particle motion inside the boundary layer are considered. A cyclone was divided into two regions, the core region and the boundary layer region, and particle motion was analyzed separately in the two regions. Turbulent dispersion in the core region was considered in calculating the particle flux distribution on the boundary layer surface, and the residual migration velocity above diffusion was introduced in modeling the deposition velocity on the wall. Over a wide range of inlet velocities and particle sizes, collection efficiency curves predicted by the new model showed an excellent agreement with experiments for low flowrate conditions (cut-sizes in the micron range) and also a good agreement for high flowrate conditions (cut-sizes in the submicron range). (C) 2001 Published by Elsevier Science Ltd.