In this research, a solar flat plate collector with a rectangular geometry with elliptical risers has was studied exptl.The collector was designed and built by the authors and the studies and tests were based on the ASHRAE standard and were done in the south of Iran.Thermal performance for water fluid and magnesium oxide nanofluid was performed in different discharges and atm. conditions.Studies were performed based on the type of fluid including ordinary water and nanofluid of water-magnesium oxide with a concentration of 1% and the environmental conditions of the test site as well as the fluid flow.The results showed that using nanofluid increases the collector efficiency in all conditions, especially at higher flow rates of the working fluid.Also, the maximum efficiency obtained in the best conditions for water is about 62.7% and for water-magnesium oxide nanofluid is approx. 82.3%.