In this paper, we investigate conjugate heat transfer by natural convection, conduction and radiation in open cavities in which a uniform heat flux is applied to the inside surface of the solid wall facing the opening. Conservation equations are solved by finite difference-control volume numerical method. The relevant governing parameters are: the Rayleigh numbers from 10 9 to 10 12 , the Prandtl number, Pr = 0.7, constant for air, the cavity aspect ratio, A = L/H from 0.4 to 1, the wall thickness l/H = 0.02-0.08, the conductivity ratio k r from 1 to 50 and the surface emissivity, e from 0 to 1. We found that the surface radiation affected the flow and temperature fields considerably. The influence of the surface radiation is to decrease the heat fluxes by natural convection and conduction while the heat flux by radiation increases with increasing surface emissivity. On the other hand, the convective and radiative Nusselt numbers are increasing functions of the surface emissivity for a given wall conductance. The combined Nusselt number and the volume flow rate are both increasing functions of the surface emissivity, particularly at high Rayleigh numbers. The convective and radiative Nusselt numbers are a decreasing function of the wall conductance and an increasing function of the aspect ratio. We found similar trends for the volume flow rate through the cavity.