Abstract: Two kinds of Nanofluids, used as recycle fluids for collectors with direct absorption of solar energy, were prepared by the “two-step method”. The photothermal conversion properties of the two kinds of nanofluids and their base fluids were studied, the temperature changing with the environmental temperature through the insolation experiment. The results show that Al-nanofluids have higher rise velocity and insolation temperature than the base fluids and Carbon Nanotube Nanofluids, and that when the insolation temperature is stable, the insolation temperature of Al-nanofluids is nearly 25 ℃ higher than that of the base fluids and 10 ℃ higher than that of the Carbon Nanotube Nanofluids. Thus, Al-nanofluids show better photo-thermal conversion properties.