Nanofluids are advantageous due to their superior thermophysical properties compared to base fluids. Increasing nanoparticle concentration increases thermophysical properties. However, viscosity enhancement results in increased pressure drop and pumping power requirement in the flow channel, which may limit the potential nanofluid applications. Because nanofluids is a relatively new subject, complexity of the nanofluid systems and nanofluid viscosity being affected by many parameters make the subject even more complicated. When literature is overviewed, lack of a general model for nanofluid viscosity is observed. In this study, nanofluid viscosity was investigated and given viscosity models were compared based on some effective parameters.