This paper presents an experimental investigation on the effect of temperature, particle size and CuO concentration on the dynamic viscosity of CuO–TEG-Water nanofluids. Various surfactants were applied for colloidal stabilization which among them just Triton X-100 exhibited the best performance. The viscosity of nanofluids that were 20 and 40 nm diameter copper oxide nanoparticles dispersed in Triethylene glycol (TEG), was measured using Brookfield rotary viscometer. The experiments were performed at 25 0C and 40 0C and particle concentration range were from 0.1 wt% to 0.9 wt%. It has been found that, in general, the nanofluid dynamic viscosity increases considerably with both of particle size and concentration. The most viscosity enhancement was about 9% for 0.9 wt% nanofluid that contained 40 nm nanoparticles at 40 0C. The results showed that the nanofluid viscosity was decreased with temperature.