Experimental investigations have been carried out on mixed convective flows induced around upward-facing, horizontal heated plates placed in a vertical downward flow of forced convection. The experiments were performed with water and air and in the ranges of the Reynolds and modified Rayleigh numbers, 300 < Re-w < 5000, 10(7) < Ra-W < 10(10) for water, and 200< Re-w < 2000, 10(6) < Ra-w < 10(9) for air. A keen interest was directed to the thermal instability of the stagnation point flows. For the sake of this, the flow fields over plates are visualized with dye and smoke. The results showed that the longitudinal vortices of which axes are parallel to the flow direction appear over the plate when the buoyancy force is beyond critical, and that the onsets of the longitudinal vortices are predicted well with the Richardson number, Gr(w)/Re-W(2) = 2.45 for both water and air. The local and overall heat transfer coefficients were also measured to investigate the influence of the longitudinal vortices on the heat transfer from the plates. The result showed that the coefficients are increased significantly from those of the stagnation point flows with the occurrence of the longitudinal vortices. It was also found that the overall Nusselt numbers of the plates are correlated well with the above Richardson number. (C) 2010 Elsevier Ltd. All rights reserved.