The maximum efficiency inlet velocity (MEIV) of the oil shale ash cyclone separator was explored in this paper. The results show that the inlet area of the separator has a significant effect on MEIV. Further analysis revealed that the cross-sectional mean axial velocity for the gas in the cylinder affording the highest efficiency was hardly affected by inlet dimensions, which circumstance could be made use of in designing the cyclone diameter. Based on the results obtained, an equation was constructed and the maximum efficiency inlet velocity for oil shale ash was predicted. Moreover, the effect of particle geometry on MEIV was ascertained.