This paper presents a numerical solution of a buoyancy-induced how and phase-change heat transfer in a vertical porous channel heated symmetrically along its vertical walls. A multiphase mixture model that accounts for complex, interacting physical phenomena such as phase change, capillary action, buoyancy-induced flour convection in the subcooled liquid and multi-dimensional effects was used. It is found that for both single and the two-phase flow with a rather low vapor fraction, the induced mass flux increases as the applied heat flux is increased. However, as the vapor fraction is increased, the numerical results show that the induced mass flux drops drastically and remains approximately constant afterwards. This result agrees qualitatively with our previous experimental study on phase-change heat transfer in a heated vertical porous tube (Zhao et al., 1998. ASME Journal of Heat Transfer, 121(3) 646-652). In this paper, the underlying mechanism leading to this interesting behavior is explained based on the liquid saturation distributions as well as the velocity fields for both vapor and liquid in the porous column.