We conducted an experimental investigation of natural convection in an enclosure containing horizontal parallel heated plates. Our experimental setup was as follows: Two heated plates were arranged vertically in an enclosure. The temperature of the ceiling was maintained at a constant low temperature, while the other sides satisfied thermal insulation boundary conditions. We examined flow characteristics in the enclosure and heat transfer around the horizontal heated plates. In the region between the cooled ceiling and the upper heated surface, vortex motion occurred when the accompanying flow from the lower region interfered with the ascending flow. Heat transfer around the heated surface was enhanced by this vortex motion. In other regions, the flow circulated along the heated surfaces in all cases. Generally speaking, for each flow pattern, the Nusselt number was proportional to some power of the modified Rayleigh number. (C) 2012 Elsevier Ltd. All rights reserved.