: In this study silica nanoparticles were synthesized by one-step process in non-aqueous environment of ethanol. These nanoparticles were preapared at the temperature of 25 ?C and the pressure of 630 mmHg at different particle mean diameters by controlling the ratio of reactants. These nanoparticles were synthesized by the method of sol-gel in which tetra ethyl ortho silicate and ethanol were used as reactants and ammonia solution as catalyst. For investigation of the effect of particle size different nanoparticle sizes of 10.6, 20.0, 38.6 and 62.0 nanometers were used. Experimentation was done at different mass fraction of 0.5, 1.5, 2.5 and 4 % and temperature of 25, 40, 55 and 70 ?C. The results showed that with the increase of particle size, the thermal conductivity of silica-ethanol nanofluid increases. Also with the increase of mass fraction and temperature the thermal conductivity increases. In this research the effect of particle size was studied on viscosity of silica-ethanol nanofluid. The results presented that with the increase in particles size the viscosity of nanofluid decreases and the nanofluid behave as non-Newtonian fluid also they showed that Power Law model is the best model for satisfying the experimental data. Results of this experimentation declare that the most efficacious mechanism for heat transfer is interfacial heat conductance, which is due to interaction of nanoparticles’ surface and base fluid molecules, in scale of microscopic heat transfer. Also they showed that the Brownian motion has less impact on heat transfer in nanofluid of silica-ethanol.