In attention to industrial significance of non-Newtonian fluids, determination of flow behaviour under various operational conditions is desirable. In this paper the isothermal and axial laminar flow of non-Newtonian fluids (power-law and Sisko) in annuli have been analyzed. In the first step the governing equations (equation of motion and rheological model) are solved with using a numerical method for power-law fluid, and the volumetric flow rate values are obtained. Then by comparing with a semi-analytical method and the methods proposed by the other workers, the accuracy of results is confirmed. Finally this numerical method is applied to Sisko fluids and the results are presented.