The transient behavior of flow boiling in vertical porous channels is numerically studied in this paper. The velocity and temperature fields under different parameters for both aiding and opposing flows are investigated. Distinctly different flow and heat transfer features are observed in the comparison of aiding and opposing flows. An analysis of liquid saturation along the heated wall indicates that the minimum liquid saturation for aiding flow is located at the tail of the heated section, whereas for opposing flow, it is within the heated section and shifts upstream with the increase of Rayleigh number and decrease of Peclet number. (C) 2010 Elsevier Ltd. All rights reserved.