In solar air heater, flat plat collectors are the best heat transfering devices. An experiment was performed with the flat plate solar air collector of area 1.87m2 to describe its transient behavior. The experiment was carried out for different range of inlet temperatures (40 oC, 50 oC, 60 oC, 70 oC, 80 oC and 90 oC) for a period of three months (April, May and June). The efficiency found for different range of input temperatures varies from 20 – 37 %.Efficiency and output temperature values are calculated theoretically and are in close agreement with experimental values. Collector time constant of the collector was also calculated.