Numerical predictions of pressure effects on natural convective heat transfer characteristics for a non-Boussinesq fluid in a rectangular enclosure are presented. The solution method is developed based on a compressible flow model and is employed to simultaneously determine the absolute pressure, density, temperature, and velocity distributions in the enclosure. Discretization equations are derived from the integral mass, momentum, and energy equations on a staggered grid. The fluid pressure in the enclosure is varied from 20 to 300 kPa such that the flow behavior in a vacuum or pressurized system can be observed. Physical situations investigated also include cases in a wide range of wall temperature difference associated with various length scales, corresponding to an equivalent modified Rayleigh number ranging from 104 to 106. The validity of the incompressible flow model coupled with the Boussinesq approximation for the fluid density, which is commonly used in the existing studies of the buoyant flows, is discussed.