A study on the heat transfer characteristics of an Air-Controlled Oxidizer(ACO) was carrier out to provide information for the safe operation of an ACO treating Depleted Uranium chip wastes(DU chips) produced in the process of nuclear fuel fabrication. Experimental have been conducted on the natural convection heat transfer in a cylindrical oxidizer with variable internal heat sources supplied by an electric heater. The rates of heat input from the heat sources were properly adjusted in accordance with the rate of heat generation estimated in the analysis of oxidation process, and the temperature profiles of the inside and outside walls, Nusselt and Rayleigh numbers were obtained. Also, the relation between Nusselt and modified Rayleigh numbers was derived.