Reduced-graphene oxide (RGO) dispersed nanofluids Were prepared by irradiating a graphene oxide (GO)/water one under UV light for different times. X-ray photoelectron spectroscopy (XPS) analyses verified the reduction from GO to RGO under the UV irradiation and indicated that the reduction degree increased with the time. Zeta potential measurements suggested that the RGO/water nanofluids obtained under the irradiation time of 340 s or less can keep stable at elevated temperatures. The transmittance of the RGO/water nanofluids was quite less than that of the GO/water one, indicating that the change from GO to RGO greatly enhanced optical absorption. The RGO/water nanofluid obtained under the irradiation of 340 s possessed higher thermal conductivity and less specific heat than the GO/water one. The good stability along with the high optical absorption and thermal conductivity make the RGO/water nanofluid exhibited superior photo-thermal conversion efficiency to the GO/water and graphene (GE)/water ones at the same loading, which reached 96.93% at 30 degrees C and 52% at 75 degrees C. It is revealed that the RGO/water nanofluid prepared from the GO/water one shows greater promise for use as the working fluid in low-temperature direct absorption solar collectors.