A TVD (Total Variation Diminishing) scheme based on a compressible Navier-Stokes equation was applied to axisymmetric flow in a tow pressure impactor with a circular nozzle, This TVD scheme was a type combined to a two-step MacCormack scheme, The shock was clearly observed at the half distance betw een the outlet of Set and the impaction plate, The critical stagnation pressure ratio of the impactor designed by Hering et al.(1)) was found to be about 0.40, The separation efficiency curve of the impactor was not dependent on the stagnation pressure ratio, nor the distance between the jet and the plate when the stagnation pressure ratio was less than 0.70.