The thermal radiative properties of nanofluids have considerable contribution to the effectiveness of harvesting solar energy. In this work, the thermal radiative properties of Al2O3-paraffin nanofluid were experimentally investigated in terms of the influence of volume concentration, nanoparticle size, temperature and optical path. The transmittance spectrums of Al2O3-paraffin nanofluid were measured by a TU-19 FTIR spectrometer which was used for the calculation for the absorption coefficient and reflectivity of nanofluid. The experimental results show that Al2O3-paraffin nanofluid performs excellent ability to absorb solar energy compared to pure paraffin, with higher absorption coefficients increased up to 15.5 times at volume concentration 0.01%. The reflectivity of nanofluid is sensitive to nanoparticle size. When the nanoparticle size decreases from 30 to 10 nm the reflectivity of Al2O3-paraffin nanofluid reduces about 37.04%. With increasing temperature, the absorption coefficient of Al2O3-paraffin nanofluid increases and the reflectivity accordingly decreases.