Due to the excellent heat transfer and transportation properties, nanofluids have brought a new direction for heat transfer and industrial application. It's highly expected that nanofluids can enhance heat transfer performance without increasing pressure drop, so viscosity is one of the key properties of nanofluids. However, researches of nanofluids are only at the beginning steps, and there aren't all-around viscosity tests on the factors and a good equation to describe the viscosity of nanofluids. Several kinds of nanofluids, varied in their nano-particle, dispersant, pH value and temperature were prepared and the viscosity was measured to analyze the trends of viscosity on all the above factors. Based on the previous viscosity models for two-phases mixture, a new model was summed up for nanofluids. Compared with test values, the calculated values on the new model have verified that the model suited to predict the viscosity of nanofluids.