In this paper we develop a pseudo function approach to obtain relative permeabilities for the numerical simulation of three-dimensional petroleum reservoirs. This approach follows the idea of an experimental approach and combines an analytical solution technique for two-phase flow with a numerical simulation technique for cross-sectional models of these three-dimensional reservoirs. The advantages of this pseudo function approach are that the heterogeneity of these reservoirs in the vertical direction and various forces such as capillary and gravitational forces can be taken into account in the derivation of the relative permeabilities. Moreover, this approach considers more physical and fluid factors and is more robust and accurate than the experimental approach. To reservoir engineers, the study of pseudo functions for the cross-sectional models of different types itself is the study of numerical simulation sensitivity of displacement processes in reservoirs. From this study they can understand the reservoir production mechanism and development indices.