Mixed convection heat transfer in rectangular channels has been investigated computationally under various operating conditions. The lower Surface of the channel is Subjected to a uniform heat flux, sidewalls are insulated and adiabatic, and the upper Surface is exposed to the Surrounding fluid. Solutions were obtained for Pr = 0.7, inclination angles 0 degrees less than or equal to theta less than or equal to 90 degrees, Reynolds numbers 50 less than or equal to Re less than or equal to 1000, and modified Grashof numbers Gr = 7.0x10(5) to 4.0x10(6). The three-dimensional elliptic governing equations were solved using a finite volume based Computational fluid dynamics (CFD) code. From a parametric study, local Nusselt number distributions were obtained and effects of channel inclination, Surface heat flux and Reynolds number on the onset of instability were investigated. Results obtained from the simulations are compared with the literature and a parallel conducted experimental study, from which a good agreement was observed, The onset of instability was Found to move upstream for increasing Grashof number. On the other]land, Onset of instability was delayed for increasing Reynolds number and increasing inclination angle. (C) 2001 Elsevier Science Ltd.