The effect of the thermal conductivity of a centered, square, heat-conducting body on natural convection In a square enclosure was examined numerically. Numerical simulations was carried out for Pr=0.17, $Ra=1.0{ imes}10^4,;1.0{ imes}10^5,;1.0{ imes}10^6$, $K^*$=1.0, 6.6, 34.0 and t=0.5, 1.0, 2.0. The results were reported in terms of streamlines, isotherms, Nusselt number. As the results, the mean Nusselt number increases with the increasing of ${zeta}$ at a constant Ra and $K^*$. In the case of ${zeta}=1.0$(obstruction shape ratio), the mean Nusselt numbers were decreased as increasing of $K^*$(obstruction thermal conductivity ratio) with regardless of the Rayleigh number. When the constant obstruction size and thermal conductivity ratio, convective heat transfer effect was more enhanced at ${zeta}=2.0$ than ${zeta}=0.5$.