Nano-composites of graphene oxide (GO) and multi-walled carbon nanotube (MWCNT) were prepared by a simple physical method and well dispersed into Therminol (R) 66 to form hybrid nanofluids with mass fractions in the range of 10 to 150 ppm. The suspension stability, optical absorption properties and photo-thermal conversion characteristics of GO/MWCNT nanofluids were tested and evaluated. GO nanosheets can avoid the self-entanglement and agglomeration of MWCNTs and ensure long-time suspension stability. The solar energy absorption capability increased with the composite concentration, and the majority of incoming radiation could be absorbed in a fluid layer of 1.5 cm. There existed an optimum concentration of 100 ppm related to the indoor experiment, and the temperature could be increased up to 94 degrees C with the corresponding receiver efficiency greatly improved from 52% (Therminol (R) 66) to 70% (100 ppm). Besides, a line focusing Fresnel lens was utilized for the outdoor solar concentrating experiment, and the photo-thermal conversion performance of hybrid nanofluids increased with the composite mass fraction from 30 to 100 ppm. A highest temperature of 153 degrees C was achieved at 100 ppm concentration. The thermal oxidation without degradation of Therminol (R) 66 occurred after indoor/outdoor experiments, and slightly enhanced the optical absorption owing to the solar radiation induced heating effect. The GO-MWCNT/Therminol (R) 66 nanofluids exhibit a good prospect in volumetric solar thermal systems from low to medium temperatures.