A numerical analysis is made to analyse the effect of uniform blowing or suction on the vortex mode of instability of a horizontal mixed convection boundary layer flow with a uniform free stream velocity in a saturated porous medium. The governing equations for the base flow are solved by using a suitable variable transformation and employing an implicit finite-difference Keller Box method. The stability analysis is based on the linear stability theory and the resulting eigenvalue problem is solved by the local similarity method. The results indicate that, for blowing, the Nusselt numbers are lower than those for an impermeable surface and the flow is more susceptible to the vortex instability, while the opposite trend is true for suction.