In this article, Cu-H2O nanofluids were prepared through two-step method. The transmittance of nanofluids over solar spectrum (250-2500 nm) was measured by the UV-Vis-NIR spectrophotometer based on integrating sphere principle. The factors influencing transmittance of nanofluids, such as particle size, mass fraction and optical path were investigated. The extinction coefficients measured experimentally were compared with the theoretical calculation value. Meanwhile, the photothermal properties of nanofluids were also investigated. The experimental results show that the transmittance of Cu-H2O nanofluids is much less than that of deionized water, and decreases with increasing nanoparticle size, mass fraction and optical depth. The highest temperature of Cu-H2O nanofluids (0.1 wt.%) can increased up to 25.3% compared with deionized water. The good absorption ability of Cu-H2O nanofluids for solar energy indicates that it is suitable for direct absorption solar thermal energy systems. (C) 2013 Elsevier Ltd. All rights reserved.