Abstract Despite the fact that Nanofluids and nanofluid based phase change materials (PCMs) have been identified as potential candidate for various heat transfer and energy storage applications, they are still facing various challenges such as stability, pressure drop and high pumping power. Nanoparticle size plays a bigger role in hindering nanofluid applications, yet effect of nanoparticles size does not receive the attention it deserves. The inconsistence and contradiction of information gives an impression that the area is not well understood therefore more information need to be made available in the literature. In this paper, the effect of nanoparticle size on thermophysical properties of nanofuid and nanofluid PCMs is reviewed. The work involves discussing the effect of nanoparticle size on thermo-physical properties of nanofluid and nanofluid PCMs through systematic analysis of past experimental and theoretical work. The results show that, thermal conductivity generally increase as nanoparticle size decreases while surface tension increases as nanoparticle size increases. Latent heat of fusion reduces as nanoparticle diameter decreases. Although, nanoparticles increase viscosity, the effect of particle size diameter is not yet clear. More research is therefore needed in this area of nanoparticle size effect on nanofluid and nanofluid PCMs so that optimum size can be established for each application.