This paper presents a systematic numerical study on a two-dimensional, steady state and laminar natural convection in a rectangular enclosure with discrete heat sources on walls. A new combined temperature scale is suggested to non-dimensionalize the governing equations of natural convection induced by multiple temperature differences. It is valuable to simplify the coupled boundary conditions and to identify the relative strength of the heat source. The heat and fluid flow structures are fully examined using streamlines and heatlines. Numerical results demonstrate the suggested combined temperature scale being efficient to characterize the various roles of the heat sources and to provide an effective approach to analyze more realistic and complex natural convection problems.