In this paper, a numerical model for three-dimensional compressible gaseous slip flow in microchannel was established. Gas viscosity is modified based on Knudsen number using Veijola's model due to the increased rarefaction effects in microscale. By comparing with experimental data, the model used in the present paper for calculating is credible. The result shows that the gas viscosity needs to be modified in the calculation because it can improve the accuracy of the result. The influences of gas compressibility, rarefaction effect and size effect on flow characteristics are comprehensively described. For compressible flow, a new non-dimensional parameter relative slip length L-s/D-h is proposed to correlate fRe. The suggested correlation for fRe with L-s/D-h is validated by comparing with experimental data. This correlation can be used for both slip and non-slip flow, and for both compressible and incompressible flow. (C) 2009 Elsevier Ltd. All rights reserved.