This work is dealing with the natural convection heat transfer in a square filled with porous medium that has been extended according to the Nield and Kuznetsov model to tridisperse porous medium. Considering impermeable walls which the horizontal ones are insulated and vertical ones are assumed to be isothermal, the governing equations are set as the three equations for momentum and three equations for energy for three phases of porosity and are numerically solved utilizing finite element method. In this study isothermal contours, streamlines and Nusselt number values are foremost criteria which are presented for three levels of porosity. The influence of various governing parameters on the heat transfer is investigated.