In this paper, Optimal homotopy asymptotic method (OHAM) and homotopy perturbation method (HPM) are applied to investigate heat transfer in the air-heating flat-plate solar collectors. Results for different values of active parameters are compared with numerical method (fourth order Runge-Kutta method). The results confirm the notion that the HPM method is more accurate than OHAM and efficient technique for finding exact solutions for differential equations which have great significance in different fields of science and engineering. Effects of air flow rate, width and length of the collector on thermal efficiency are examined. As an important outcome, increasing in the collector's dimensions (width and length) make a decreasing in thermal efficiency, but increasing in air mass flow rates improve it.