Transient laminar free convection in a water-filled rectangular enclosure has been studied experimentally and numerically. The study aimed at gaining an insight into the physical phenomena, illustrated by the flow patterns and the temperature stratification, from the initial stage of a homogeneous fluid through the transient evolving process. The experimental setup consisted of two rectangular water-filled enclosures: the smaller one contained a heat source and was centrally immersed in the larger one. Dye was injected for visualization of the rising boundary layer over the inner container walls, and the accumulation of the colored layers in the upper zone of the outer container. Simultaneously, the temperature measurements, by thermocouples and an infra-red camera, displayed the formation of a stratified structure. The numerical study was conducted three-dimensionally, using the Fluent 6.3 software. The stratified temperature field, observed experimentally, was well-predicted by the numerical model. The heat transfer rates and the transient averaged Nusselt numbers were compared with the data in the literature. The current, constant power input yielded Nu/Ra0.25∼0.7 in the range of Fo/Ra0.25 = 0-3.