In order to enhance thermal efficiency of a flat plate solar collector, the effects of thermo-physical properties of short Single Wall Carbon Nanotubes (SWCNTs) suspended in water was investigated in this study. Sodium dodecyl sulphate was used as a dispersant for preparing a stable nanofluid. Subsequently, the nanofluid was comprehensively characterized by particle size measurement and spectroscopic technique. Specific heat with the increase of particle loading and temperature was investigated. Thermal conductivity increment also showed a linear dependence on particle concentration and temperature. Viscosity of the nanofluids and water reduced with the increase of temperature and increased with the particle loading. Using improved thermo-physical properties of the nanofluid, the maximum energy and exergy efficiency of flat plate collector was enhanced up to 95.12% and 26.25% compared to water which was 42.07% and 8.77%, respectively. This low exergy efficiency shows that flat plate collectors still require substantial enhancement. To the authors’ knowledge, SWCNTs–H2O was used as the functioning fluid for the first time to investigate both the thermos-physical properties as well as the increase in thermal efficiency of a flat plate solar collector.