The vortex instability of laminar mixed convection flow over isothermal, inclined flat plates is investigated by the linear stability theory. Critical Grashof and Reynolds numbers that predict the first occurrence of vortex rolls are obtained for fluids having a Prandtl number of 0.7 under various buoyancy force intensities and over a range of angles of inclination. It is found that at a given angle of inclination the flow becomes more susceptible to the vortex mode of instability as the buoyancy force increases. However, for a given bouyancy force intensity, the susceptibility of the flow to the vortex mode of instability decreases as the plate is tilted from the horizontal toward the vertical orientation, finally attaining the absolutely stable condition when the plate is vertical. The results are also compared with analytical results for the wave mode of instability.