Abstract: In order to investigate the special phenomena and laws of natural convection for the nonBoussinesq fluid in rectangular cavities, a series of steady twodimensional numerical simulations on natural convection of cold water in a rectangular cavity were conducted by using finite volume method. The flow and temperature fields for different aspect ratios of the cavity and Rayleigh numbers were obtained. The mean Nusselt numbers for different conditions were also compared and analyzed. The results show that the flow is enhanced and the number of cells increases with the increase of the aspect ratio of the cavity. The mean Nusselt number increases accordingly and finally approaches a constant value. With the increasing of the Rayleigh number, the heat transfer is enhanced and the transition of the flow structure will occur under smaller aspect ratio. Based on the simulation results, the heat transfer correlation were proposed according to the multiple linear regression.