The linear stability analysis of mixed convection in a heated vertical channel filled with a porous medium was investigated. The Galerkin method was employed in this calculation. The results indicated that the fully developed laminar flow could become unstable under a mild heating condition for higher modified Darcy numbers of Da* = 1 and 10(-2). but the critical Rayleigh number increases substantially for Da* = 10(-4). The disturbance kinetic energy budget analysis showed that, for Da* = 1 and 10(-2), there are generally three instability types, the thermal-buoyant, mixed, and thermal-shear instabilities for low, intermediate, and high Reynolds numbers, respectively, while for Da* = 10(-4), the thermal-buoyant instability is the only driving mechanisms of flow instability regardless of the value of Reynolds number. (C) 2003 Elsevier Ltd. All rights reserved.