Experimental study was carried out on the temperature profile of natural convection of air in a 50 mm cube with the temperature difference of 30 K between two opposing vertical walls. The objective is to investigate the essential aspect of heat transfer in the wide range of Rayleigh number (Ra) by lowering the pressure. The pressure was varied from 5.40 kPa (40.5 mnHg) to 99.99 kPa (750.0 mmHg). These correspond to Ra=1.04x10(3) to 3.56x10(5). The results show that the temperature distribution and Nusselt number approach those of conduction state as the pressure decreases. (C) 2003 Elsevier Ltd. All rights reserved.