A numerical study has been conducted to investigate the effects of wall thickness and wall thermal conductivity on natural convection heat transfer in an externally heated square enclosure having finite wall conductance and wall thickness. The interaction between convection in the fluid filled cavity and conduction in the walls surrounding the cavity is investigated. The studied parameters are wall-fluid thermal conductivity ratio k* ( k w / k f ), wall to height thickness ratio t* ( t w / L ), and Rayleigh number Ra. Steady state, two dimensional natural convection problem is solved using finite volume code (Fluent 6.2). Results are presented in terms of streamlines and isotherms, and total heat transfer rate Q*. Results have been obtained for Rayleigh numbers (Ra) between 103 and 106, wall thickness ratio (t*) between 0.005 and 0.2 and wall-fluid thermal conductivity ratio (k*) between 1 and 100. Results reveal that heat transfer rates across enclosure are complex functions of wall thickness ratio t*, Rayleigh number Ra, and the thermal conductivity ratio k*. Results indicate that conduction along the enclosure walls has a stabilizing influence on the convective motion in the enclosure and is therefore responsible for reduced average temperature differences across the cavity. The total heat transfer rate is correlated with Ra for different k* and t*. The isotherms and streamlines are produced for various Rayleigh numbers and geometrical conditions. It is found that the total heat transfer rate is an increasing function of the Rayleigh number, thermal conductivity ratio k* and a decreasing or increasing function of the wall thickness ratio t* according to the values of k* and Ra.