This experiment on natural convection heat transfer with the PCM microcapsule slurry for a solid phase state of the PCM were performed at a horizontal rectangular enclosure heating from below and cooling from top. In the present study, important parameters affecting the natural convection heat transfer of the slurry with the PCM were taken into account such as the mass concentration of the PCM microencapsulated into the slurry, the temperature difference between heating plate and cooling plate, the Rayleigh number Ra and the aspect ratio (width/height) of the horizontal rectangular enclosure. The heat transfer characteristics of the PCM microcapsule slurry on natural convection were concluded from predicted conditions simplified by classifying the temperature ranges into three sub-regions, in which the PCM in the slurry was in only solid phase state, phase change state or solid-liquid coexistence state and only liquid phase state of the PCM in the microcapsule, respectively. The upper cooling plate was fixed at a constant temperature in each region, and the lower heating plate temperature was varied by carefully adjusting the electric power input of an electric plate heater. And experiment was done under the thermal steady condition in the PCM microcapsule slurry. Emphasis was given on phase changing temperature range, and experiments had been performed for four kinds of enclosures with various heights.