The object of this research is studying the viscosity of nanofluids and the effective factors on it such as temperature, shear rate, concentration of nanotubes, sonication time and the composition of base fluid. In this project, carbon nanotubes have been used for synthesis of nanofluids with binary mixture of ethylene glycol and propylene glycol. Nanofluids have prepared with chemical treatment and ultrasonic homogenization. To prepare more stable nanofluids, samples are sonicated at optimum sonication time. The results show that the viscosity of the nanofluids increase with sonication time until it reaches toa maximum and then decreases.Whereas this increase and decrease in viscosity is due to breaking the aggregates of MWCNTs and bricking of them, respectively, optimum sonication time has the maximum value of viscosity. Because with bricking and shorting of anotubes, the aspect ratio of them decreases that caused to decrease in thermal conductivity of nanofluids. We have shown that with increasing the concentration of nanotubes, not only the viscosity of nanofluids increase, but also the optimum sonication time increases. Viscosity measurements of nanofluids one month show that the stability of nanofluids decreases with loading of nanotubes. Also, viscosity decreases with temperature that is due to decrease in shear stress that cussed to the fluid's layers move easily therefore the viscosity of fluids decreases.This results show that in low concentration of nanotubes the relative viscosity of nanofluids is independent of temperature but with increase in concentration of nanotubes, dependence of viscosity to temperature increase, too. We have shown that the viscosity of base fluid is invariant with shear rate. But it is notable that at a fixed concentration of CNT, viscosity of nanofluids decreased with shear rate that is due to the decrease in shear stress. However, nanofluids have been shown a shear thinning behavior. Therefore the non-Newtonian behavior of nanofluids arises from the aggregation of CNTs in the samples. Moreover, these results show that with increase in concentration of nanotubes, dependence of viscosity to shear rate increase. In addition, we have studied viscosity of these nanofluids with composition of base fluid. Our results show that the viscosity of nanofluids increase with volume fraction of PG in the base fluid. Also Because of the decreasing in polarity of base fluid at high volume fraction of PG, dispersibility of CNTs decreased and therefore, the optimum sonication time increased. Moreover, it has been found that dependence of viscosity to shear rate and temperature increase at high volume fraction of PG in base fluid. These results also show that nanofluids with high volume fraction of PG are more stable.