Abstract: Experiments have demonstrated that when nanoparticles such Al2O3, Cu, TiO2 are added to traditional heat exchanger working fluids their thermal fluid properties changes. Especially, the effective thermal conductivity of the nanofluids increases. Different empirical models were suggested to predict there effective thermal conductivity based on experimental data. Effective thermal conductivity varies with size, shape, volume fraction of the nanoparticles and thermal conductivities of the particle and base fluid. Hence, this study investigates the effect of nanoparticles size, volume fraction and type of nanoparticles in water based nanofluids. Using interfacial shell concept, the Hamilton and Crosser model was modified to study the nanoparticles size effect. The modified model was compared with literature data and found under predict the effective thermal conductivity but follow the same trend. The result shows that as the nanoparticles size increases the thermal conductivity decreases. Cu water nanofluid gives better thermal conductivity than Al2O3 water nanofluid at a given particle volume fraction.