In this study, effects of multi walled carbon nanotubes and temperature on rheological behavior of engine oil (10W40) have been examined. For this purpose, the experiments were carried out in the temperature range of 5-55°C for several suspensions with solid volume fractions of 0.025%, 0.05%, 0.1%, 0.25%, 0.5% and 0.75%. The viscosity of all samples was measured in the shear rate range of 666s-1 to 13333 s-1 at all temperatures considered. The viscosity measurements at different shear rates revealed that all nanofluid samples showed non-Newtonian behavior. The results also revealed that for an increase in the solid volume fraction from 0 to 0.75%, the viscosity increases to 2.5 times. The consistency and the power law index were attained by curve-fitting method for all samples and temperatures. Furthermore, the curve-fitting results revealed that the consistency index and apparent viscosity of nanofluid increases with augmenting the solid volume fraction and diminishes with growing temperature.