CFO(Critical Flow Orifice), micro sonic nozzle, is a device in tobacco industry to give a standard gas flow rate. Adopting the blocking effect of the convergent nozzles for compressible fluid, CFO can produce an accurate flow rate in pipe line. CFO can be used in micro flow calibration such as in the measurement of the pressure drop of filter rod or draw resistance of cigarette. To study the effect of convergent shapes on the aerodynamic characteristics of CFO, the 3D numerical simulations are carried out on two kinds of CFO structure shapes with finite volume method, based on the equations of compressible gas. The adopted turbulence model is the realizable k-ε model. According to the results of simulation, using Vyshinsky curve as the convergent curve is better than to apply the cone-shaped curve with the same exit diameter of nozzle. This is difficult to be observed in the experiments. The influence of convergent shapes can provide a basis for optimization design of CFO.