This paper studies the combined radiation and natural convection in an open vertical cylinder. The cylinder is heated through its sidewall at a constant heat flux and cooled at the top surface via radiation. The high order finite difference method is used to simulate the fluid flow and heat transfer inside the cylinder and the internal radiation is solved using discontinuous finite element method. The two numerical methods are coupled through an iterative process. Based on the numerical simulations, a linear stability analysis is carried out to investigate how the internal radiation changes the stability of the flow.