Numerical analysis of thermally developing and simultaneously developing mixed convection flow and heat transfer with variable viscosity in an isothermal horizontal tube has been carried out. Parametric computations were performed to investigate the effect of inlet Prandtl number, inlet Rayleigh number, wall-to-inlet temperature difference, and inlet axial velocity profile on the Nusselt numbers and apparent friction factors for both heating and cooling conditions. The results indicate that the effect of variable viscosity is more pronounced on the friction factor than on Nusselt numbers. in addition, the effect of inlet Rayleigh number and inlet velocity profile on the Nusselt numbers and friction factors exists only in the near-inlet region. A parameter found by scaling analysis was used to empirically correlate the. computed Nusselt number and the friction factor data and the available experimental Nusselt number data for both thermally developing and simultaneously developing flow and heat transfer. The developed correlations are more accurate, have wider ranges of applicability than those available in the literature, ana should be of much use to designers.