Solar thermal technology has gained importance during its globalization and industrialization, due to its effective conversion of solar energy into thermal energy. Common flat black surface solar collectors generally employ selective solid coating as a solar radiation absorber; nevertheless, these collectors have several shortcomings such as low efficiency, corrosion, heating losses and cannot withstand high incident flux. Theoretical and experimental studies have shown that the nanofluids based Direct Absorber Solar Collectors (DASC) can harvest solar energy more efficiently compared to the conventional solar collectors. Present work reports the investigation of dispersion stability, optical and thermal properties of partially unzipped multiwalled carbon nanotubes (PUMWNTs) based nanofluids for DASC. PUMWNTs were prepared by modified Hummers method. Nanofluids were prepared by dispersing calculated amount of PUMWNTs in DI water and ethylene glycol (EG). Absorption and transmittance studies were conducted using UV-vis-NIR spectrophotometer. Absorption spectra confirmed the stability of nanofluids. The extinction coefficient of nanofluids shows significant improvement as compared with that of base fluids even at low concentration. Furthermore, the temperature dependent thermal conductivity study with different volume fractions carried out for DI water and EG based nanofluids reported considerable enhancement of 27% and 20.97% respectively. Based on enhanced optical and thermal properties, PUMWNTs dispersed nanofluids are found to be promising to increase the overall efficiency of DASC. (C) 2016 Elsevier B.V. All rights reserved.