The role of inertia effects is studied in the flow and vortex instability of a:horizontal natural convection boundary layer flow in a saturated porous medium with the influence of surface mass flux. The numerical results of the velocity and temperature profiles, Nusselt number, critical Rayleigh number and associated wave number at the onset of vortex instability are presented for various values of the Forchheimer coefficient and the surface mass flux. The results demonstrate that the Forchheimer coefficient reduces the heat transfer rate and destabilize the flow. Further, the decreasing values of suction reduces the heat transfer rate and stabilize the flow. (C) 2004 Elsevier Science Ltd.