This paper presents the experimental study of a flat plate solar collector to find the optimum flow rate of collector with variation of daily solar radiation intensity. The procedure of ASHRAE standard was used for testing the thermal performance of flat-plate solar collector. Results show that using optimal collector flow rate can improve energy and exergy efficiency of SWH. Furthermore, results show that in no load on storage tank overall energy efficiency can improve between 0.25 % and 7 %. In addition, calculations show an increment between 0.18 % and 1.12% for overall exergy efficiency. In the 36 L/h, and 52 L/h and 72 L/h thermal load on storage tank overall energy and exergy efficiency show a reasonable increment with using optimum flow rate of collector. Finally, these experimental analysis results can be a base for design a collector flow rate controller to have a more efficient SWH.