Experiments were conducted on a solar flat plate collector (FPC) with graphene oxide (GO) nanofluid as the working fluid, circulated forcibly with the aid of a pump. The FPC had a collector area of 2 m(2), a concentric tube heat exchanger (CTHX) and a storage tank. GO nanoparticles were synthesized from graphite by the modified Hummer's method. The morphology of the GO was estimated by XRD, UV-vis spectrometry and SEM imaging. The GO nanofluid was prepared by ultrasonication of the GO nanoparticles with de-ionized (DI) water as the base fluid, without adding any surfactant. Homogeneous and stable GO nanofluid for three different mass concentrations, (phi) = 0.005, 0.01 and 0.02, were prepared. The nanofluids were found to be stable for 60 days without any sedimentation issues. The thermo-physical properties of the GO nanofluids, such as the thermal conductivity, viscosity, density and specific heat, were estimated. For the flow supplied by the pump, the maximum value of the FPC inlet pipe and fluid property-based Reynolds number was found to be 430. The FPC's overall heat transfer co-efficient (h), friction factor and collector efficiency were investigated for GO nanofluids under laminar conditions. It was observed that for GO nanofluid with phi = 0.02 and a flow rate of 0.0167 kg/s, the enhancement in the collector efficiency was 73% over that of the base fluid (DI water). The collector efficiency was found to increase with increasing phi and flow rate. The increases in h for the GO nanofluid with phi = 0.005, 0.01 and 0.02 were 8.03%, 10.93% and 11.5%, respectively. (c) 2016 Elsevier Ltd. All rights reserved.