It is the aim of the present paper to investigate the effect of Graphene nanofluid on thermal performance of flat plate solar collectors. Therefore, the influence of Graphene Nanoplatelets on thermal efficiency of a closed-cycle solar system consisted of a flat plate solar collector has been investigated and its hot water production capability has also been tested. To start, the structure of Graphene has been evaluated by Field Emission Scanning Electron Microscopy imaging and UV-vis spectroscopy. Afterward, by utilizing deionized water as the base fluid, Graphene/Water nanofluid has been prepared by a two-step method with different mass fractions (0.01 and 0.02 wt%). To prevent from sedimentation, the colloidal stability of the nanofluid has been tested in different pHs and its best level has been obtained. Moreover, the thermal conductivity, kinematic viscosity and the influence of Graphene/Water nanofluid on the efficiency of the flat plate solar collector have been studied thoroughly. The results indicate that dispersing Graphene in the base fluid can increase thermal efficiency of the solar collector up to 18.87%. As an obvious result, the outlet temperature and absorbed energy of water heater in various states (deionized water, 0.01 and 0.02 wt% nanofluid) has been measured and the temperature of water in the solar heater reached up to 71 degrees C. Eventually, the theoretical thermal efficiency of the solar collector has been calculated and compared with the experimental one, and the results have been closed to each other. (C) 2016 Elsevier Ltd. All rights reserved.