In order to evaluate the influence of extremely low particle concentration oil separation performance of cyclone separator, the overall collection efficiencies and grade efficiencies of a cyclone separator with particle concentrations of 5-2000 mg/m(3) and inlet velocities of 6-30 m/s have been investigated under ambient temperature and atmospheric pressure conditions. Aerosol spectrometer based oil measuring particle number is used to measure the particle concentrations and particle size distributions of the inlet and outlet of the cyclone separator. The overall efficiency is equal to the ratio of the particle concentration difference between the inlet and outlet of the cyclone separator to the inlet particle concentration. The grade efficiency is obtained by comparing the particle size distributions of the inlet and Outlet of the cyclone separator. The effects of particle concentration on separation performance are predicted by Smolik empirical model. Particle agglomeration, which has been found in the inlet and Outlet of the cyclone separator, has a very important influence on the collection efficiencies and grade efficiencies of the cyclone separator at the particle concentration of 5-2000 mg/m(3). The cut sizes for different inlet gas velocity with extremely low particle concentration can be quantitatively Calculated by Barth model, Mothes and Loffer model and Muschelknautz model, respectively. Experimental results show that the overall collection efficiencies and grade efficiencies increased with the increasing particle concentrations and inlet velocities, and most of the particles with the diameter bigger than 10 mu m can be removed by cyclone separator. (C) 2008 Elsevier B.V. All rights reserved.