The interaction of natural convection with thermal radiation in three dimensional rectangular cavities with gray surfaces is studied numerically. The cavity is heated and cooled from the vertical opposing isothermal walls while all other side walls are adiabatic. The pins are placed to the active wall. The fluid is nonparticipating air (Pr=0.71), and the flow of air is assumed to be laminar. The fluid properties are taken to be constant at mean values, except for the density for which the Boussinesq approximation is employed. The governing flow, energy and the radiative transfer equations are solved by a commercial CFD code-ANSYS. The convection terms are discretized using a second order upwind scheme, and SIMPLE algorithm is adapted as the solution algorithm. The surface to surface (S2S) model is used for radiative heat transfer simulations. The Rayleigh number (Ra), the surface emissivity (epsilon), the number of pins, the dimensionless pin length and diameter (B=b/H and D=d/H), the slenderness of the cavity (W/H) are variable parameters of this study. The slenderness ration of W/H=0.5, 1.0 and 2.0 are considered. Enclosure with pin arrangements of 9, 13, 17, 21 and 25 are considered. The Rayleigh number, the surface emissivity, the dimensionless pin lengths and diameters are varied in the following intervals 10(3)<= Ra <= 10(6), 0.0 <=epsilon <= 1.0, 0.05 <= B <= 0.6, 0.025 <= D <= 0.1, respectively. The mean Nusselt numbers for convection and radiation, as well as total, were computed as a function of the Rayleigh number for each case. Also the flow and temperature distribution for some cases have been presented and discussed.