In this study, the flow and heat transfer characteristics and the performance of double-pass flat-plate solar air heaters are studied numerically. The effects of the total air mass flow rate and the fraction of air mass flow rate in the upper or lower flow channel on the performance of the double-pass flat-plate solar air heaters have also been investigated numerically. The mathematical models that describe the flow and heat transfer characteristics of the double-pass flat-plate solar air heaters are derived from the momentum and energy conservation equations. The developed mathematical model has been solved using a finite volume discrete scheme utilizing the SIMPLE (Semi-Implicit Method for Pressure Linked Equations) pressure-correction scheme combined with QUICK (Quadratic Upwind Interpolation Convective Kinematics) scheme in the frame of staggered system. The results obtained from the numerical model are validated by comparison with experimental data of previous researchers. It has been found that there is a reasonable agreement between the present model results and those obtained by previous investigators.