The viscosity of ferrofluid has a vital role in liquid sealing of the hard disk drives, biomedical applications as drug delivery, hyperthermia, and magnetic resonance imaging. The theoretical expressions for ferrofluid viscosity and its limitation are presented in detail in this article. A qualitative comparison of the theoretical and experimental viscosity results is also shown. In the absence of a magnetic field, the viscosity of ferrofluid depends on the volume concentration of magnetic nanoparticles, including surfactant layers. However, the viscosity of ferrofluid due to stationary magnetic field depends on the angle between the applied magnetic field and vorticity in the flow. If this angle is 90 deg, then there is a maximum increase in the viscosity. When field frequency matches with the relaxation time, known as resonance condition, then there is no impact of an alternating magnetic field in the viscosity of ferrofluid. If the frequency of an alternating magnetic field is less than resonance frequency, then an alternating magnetic field increases the viscosity of ferrofluid. For diluted ferrofluid, present theoretical results of viscosity have good agreement with the experimental data.