Two-dimensional slot jet impingement cooling of an isothermal horizontal surface immersed in an unconfined porous medium is simulated numerically to gain insight into thermal characteristics under mixed convection conditions with the limitation of the Darcy model. The jet direction is considered to be perpendicular from the top to the horizontal heated element; therefore, the jet flow and the buoyancy driven flow are in opposite directions. The results are presented in the mixed convection regime with wide ranges of the governing parameters: Peclet number (1 <= Pe <= 1000), Rayleigh number (10 <= Ra <= 100), half jet width (0.1 <= D <= 0.5), and the distance between the jet and the heated portion (0.1 <= H <= 1.0). It is found that the average Nusselt number increases with increase in either Rayleigh number or jet width for high values of Peclet number. The average Nusselt number also increases with decrease in the distance between the jet and the heated portion. It is shown that mixed convection mode can cause minimum average Nusselt number at two values of Peclet number and a maximum average Nusselt number occurs in between theses two Peclet numbers at higher Rayleigh number due to counteraction of jet flow against buoyancy driven flow. Hence careful consideration must be given while designing a system of jet impingement cooling through porous medium. (C) 2011 Elsevier Ltd. All rights reserved.