The gas phase flow fields in the geometrically similar cyclone separators are simulated by Fluent 6.1.The cyclone diameter in the simulation ranges from 100 mm to 2100 mm.Based on the simulation results，the effect of cyclone size parameters on the dimensionless velocity similarity is investigated.The results show that the flow fields are similar for different cyclone diameters，but the dimensionless tangential velocities are different.Hence，the flow field is not strictly similar.As the cyclone diameter increases，the flow field develops from weak similarity to strong similarity.When the cyclone diameter is more than 2000 mm，the dimensionless velocities are constant，indicating that the Eu number is not affected by Re number and the flow field is in self-similarity.The self-similarity of the flow field reflects the contribution of the inertia resistance loss and viscous friction loss to the total energy loss.As the cyclone diameter increases，the viscous friction loss decreases while the inertia resistance loss increases，but the Eu number decreases.When the cyclone diameter increases to some value，Eu number is independent of Re number，and the flow field is in self-similarity.