Combined radiation and natural convection within annular spaces and sectors are investigated numerically. The medium is considered as gray, emitting, absorbing, and isotropically scattering gas. The finite volume radiation solution method and the control volume approach are used to discritize the coupled equations of momentum, energy, and radiative transfer. The effects of the boundary conditions (for annular sectors) and the position of inner cylinder (for annular spaces) on the intensity and the stability of the flow are analyzed. Numerical solutions are obtained fo Pr = 0.7, Ra ≤ 2·10 5, and the radiation-conduction parameter ranging from 0 to ∞.