Abstract- The present study aims to investigate numerically the natural convection of various nanofluids inside a square enclosure with a central heat source at different aspect ratio. Also, some correlations are presented in order to calculate the Nusselt number in terms of Rayleigh number and volume fraction of nanoparticles. The heat source and cavity walls are kept at constant temperatures of Th and Tc, respectively. The nanofluids are considered to be water as the base fluid and different nanoparticles such as Cu, CuO, Ag, Al2O3, or Tio2. To discretize the governing equations, the control volume method and SIMPELER algorithm have been employed. The study has been carried out for aspect ratios from 0.2 to 0.8, Rayleigh numbers from 1e3 to 1e6 and the volume fractions of nanoparticles ranging in 0-0.05. The results indicated that the Nusselt number increases with increasing the volume fraction of nanoparticles as well as the aspect ratio. Furthermore, by increasing the Rayleigh number, some eddies, of kind of Rayleigh-Benard, are developed in the space between the heat source and the upper wall of the enclosure. Based on the obtained results, several correlations with high accuracy have been present in order to evaluate the Nusselt number.