In this paper, the performance analysis of solar air heater with triangular longitudinal fins has been analyzed theoretically. The flowing channel is formed by two diagonally positioned triangular fins attached on the absorber plate with bottom side thermally insulated and the top surface of absorber plate subjected to unvarying heat flux. The expression for collector efficiency factor of such collector has been established. Effects of mass flow rates and varying solar flux on the thermal performance have been presented and the results are compared with plain solar air heater and rectangular finned solar air heater. Various factors versus mass flow rate and solar flux graphs were plotted. The mathematical model provides reasonable indication of the performance of triangular fin solar air heater and it can be useful in designing such types of solar airheater.