In this study an experimental work has been conducted to investigate the natural convection through a rectangular enclosure fixed horizontally. The upper surface for the apparatus has been heated and cooled. The test apparatus has been manufactured first, and then thermocouples have been fixed in proper positions. Then many readings, for temperatures, have been registered for each thermocouple. These readings have been taken for different mass flow rates of cooling water with different heat fluxes. These experiments have been conducted for the range of Rayleigh number , and for the range of water mass flow rates in cooling region (0.009 kg/s≤ ≤0.04 kg/s). Four tests for each mass flow rates of water have been conducted, for each one of them five values of heat flux have been used in heating. It has been noticed that the surface temperature of experiments section increased to approach from heating region and the same behavior of temperature inside the section, with appearance stagnation region and not change of air temperature on center region for the apparatus. The study shows that Nusselt number decreases by (50.4 %) in cooling region, and Nusselt number value increases by (33%) in heating region as the mass flow rate of water decreases by (77.5%) for the highest and lowest heat flux. Also, Nusselt number decreases by (57.5%) as the heat flux decreases by (95%) for the highest and lowest water mass flow rate. An experimental correlation has been adopted between average Nusselt number against average Rayleigh number (Ra) for both heating and cooling region is .