An air filled rectangular enclosure containing an isothermal plate is cooled from a lateral wall while other three sides arc insulated. A horizontally situated thin isothermal plate is the sole source of heat input within the system. The plate and the cold walls are maintained at constant temperatures. The transport equations, along with the Boussinesq approximation as well as the energy equation. are solved using the finite Volume method (FVM) coupled with SIMPLE, algorithm. The stead), state mean Nusselt number over the isothermal plate surface area is computed for each case as a function of Rayleigh number, the plate length and position. Rayleigh number is varied From 10(5) to 5 x 10(7) while two plate length alternatives were studied-50%. and 75% of the enclosure length. The plate thickness was kept constant-1% of the enclosure height. The effects of the plate length, the plate position and the aspect ratio (A=1 and 2) on the heat transfer characteristics and the fluid flow were investigated. For increasing Rayleigh number the heat transfer rates increases. and for increasing plate length the heat transfer rate (Nusselt number) decreases by about 25%. As a result of the numerical parametric study, a correlation for the Nusselt number is obtained to be used for practical cooling problems.