Present study is aims at a comprehensive review of Nanofluid heat transfer mechanism and thermo-physical properties. The Nanofluid preparation techniques, heat transfer mechanism and thermo-physical properties are discussed in detail. Heat transfer in Nanofluid is function of many parameters, like Brownian motion of particle, molecular layering of the liquid-particles interface and nanoparticle clustering etc. and these parameters are discussed in detail. Also metal oxides nanoparticles had a great effect in heat transfer enhancement of the nanofluid. The coolants have high thermal conductivities at low volume concentration of Nanofluid. Nanofluids’ thermal conductivity and viscosity greatly depend on their size, volume concentration, shape and material. Nanofluid viscosity may increase or decrease depending upon the material of nanoparticle on lowering the size. Viscosity of nanofluid increases with increase in volume fraction of nanoparticle. © IAEME Publication