This paper describes a practical thermal design approach to natural air-cooled electronic equipment casings. A set of simplified equations for the thermal design of natural air-cooled electronic equipment casings has been proposed. The proposed set of the equations satisfied the demand of practical air cooling systems, since it takes account of the factors such as stack effect, air flow resistance, heat transfer due to natural convection and so on. The effects of the outlet area and the location of the main power dissipation unit on the natural cooling capability of electronic equipment casings were studied using a set of equations. The results have shown that a uniform temperature distribution could be achieved in the case of placing the main power dissipation unit at the bottom of the casing. It has also been suggested that the outlet vent area should be limited within some threshold value so that the value of heat removed from the casing surface would be higher than that from the outlet vent.