The flow fields in cyclone at various model gases with ρ=0.6125~2.4500kg/m3 and μ=4.4735×10-6~4.4735×10-5kg/(m〖DK〗·s) were simulated by the computational fluid dynamics (CFD) software package, FLUENT 6.3. The influences of density and viscosity of gases on the dimensionless tangential velocity and pressure drop were performed respectively by single factor analysis. The result indicated that the dimensionless tangential velocity of gas flow in cyclone increases as the viscosity increase, while decreases with the increase of viscosity. Both density and viscosity have logarithmic function relations with the dimensionless maximum tangential velocity. The drag coefficient is not only influenced by the geometric parameters of the cyclone, but also affected by the viscosity of gases; there is a logarithmic regression between the pressure drop and viscosity of gas. Moreover, a fitting bivariate polynomial equation between the dimensionless tangential velocity and the factors was obtained.