In this study, mixed convection in vertical and inclined parallel plate microchannels, has been investigated. The flow is steady, laminar, and incompressible and the walls are at constant temperature. A FORTRAN program has been written to solve the governing equations with wall velocity slip and temperature jump boundary conditions, and the results have been presented for Reynolds numbers of 25, 50, and 100, Knudsen numbers of 0, 0.005, 0.02, and 0.09, Richardson numbers of 0, 1, and 1.77, and microchannel inclination angles of 10°, 30°, 50°, and 90°. Results showed that wall velocity slip, friction coefficient, centerline velocity, and heat transfer rate decreases with increasing the Knudsen number. Increasing the Richardson number, for a given Reynolds number, decreases the microchannel centerline velocity because of the increasing of wall adjacent velocity, which is due to the promotion effect of natural convection. Also, the friction coefficient and Nusselt number increases with channel inclination angle.