Summary: The present contribution deals with the thermophoresis particle deposition effect on the free convection over a horizontal flat plate embedded in a fluid-saturated porous medium. In a certain sense, this study continues that by Chamka and Pop [A. Chamka, I. Pop, Effect of thermophoresis particle deposition in free convection boundary layer from a vertical flat plate embedded in a porous medium, Int. Commun. Heat Mass Transfer 31 (2004) 421-430], where the vertical flat plate configuration was analyzed. The governing equations are transformed into a set of coupled differential equations, which are solved numerically using a finite difference method. For various values of the problem parameters, graphs of the profile concentration in the boundary layer and of thermophoretic deposition velocity are presented.