This study applies thermal plume theory and laboratory water tank experiments to investigate the effect of a localized heat source at different levels on buoyancy-driven flows in a naturally ventilated space. The space has an inlet and outlet openings connecting to the exterior environment respectively at the floor and ceiling levels, and keeps a constant heat source at several levels inside it. In the laboratory experiments, the thermal structures were measured by a series of thermocouples and the flow fields were captured by a laser light sheet flow visualization method. The research results show that a different heat source level changes the mass flux in the space, and also the temperature field and the stratification interface of the buoyant layer. When the same heat strength source is raised to a higher level, the mass flux decreases, but the temperature increment and the interface of the buoyant layer both rise in the steady state. In the same space, an elevated heat source level always leads to a higher interface level regardless of the input heat strength, and the stratification interface changes more linearly with the heat source level for the case of a smaller effective opening area. The stratification performance of buoyancy-driven displacement ventilation by a heat source at an elevated level is different from that of mechanical displacement ventilation with the same heat source condition. (C) 2012 Elsevier Ltd. All rights reserved.