Viscosity of Fe 3 O 4 /Ethylene glycol magnetic nanofluid with coating of polyvinylpyrrolidone (PVP) as nanoparticles, under DC and AC magnetic field was investigated. Viscosity is an important property of fluid that has direct effect on pumping force. Beside of that, viscosity influences in Prandtl number, so it can affect heat transfer of fluid. The most of previous studies have considered the heat transfer properties of nanofluids, and there are a few works about the rheological behavior of them. In this project, at the first, rheological behavior of this nanofluid in different volume fractions of nanoparticles have been characterized by Brookfield rotational viscometer. Results showed a Newtonian behavior for these nanofluids. Therefore a capillary tube viscometer could be used confidently for measuring of viscosity of this nanofluid. So a handmade capillary tube viscometer was used for measuring the viscosity since the magnetic field could be applied by using this viscometer. In order to obtain the accuracy of capillary tube viscometer, viscosity of base fluid was measured in different temperatures and compared with source data. Fe 3 O 4 nanopaticles were dispersed in Ethylene glycol and nanofluids prepared with volume fractions of 0.005, 0.01, 0.015 and 0.035%. According to results obtained, viscosity of nanofluid reduced with concentration of nanoparticles. Every experiment has repeated for four times and standard deviations (SD) of each data were obtained. The greatest reduction occurs for volume fraction of 0.035%, which was 20.1% respect to base fluid. It seems that this viscosity decrement is due to interactions between coatings of nanoparticles (PVP) with ethylene glycol. Applying DC magnetic field on nanofluid in the parallel direction of fluid flow showed that the viscosity of magnetic nanofluid decreases with strength of magnetic field. This reduction was maximum for nanofluid with volume fraction of 0.015%. Magnetic force, make magnetic particles to became dipole and by overcoming the displacement movement of particles make them to organize chains. These chains influence the viscosity of magnetic nanofluid. With increasing in time of applying magnetic field, at first viscosity decreases, and after that, viscosity approaches to a constant amount due to reach to saturation magnetism status for magnetic nanoparticles. The parallel magnetic field was applied by a solenoid. Changing the direction of parallel magnetic field didn’t have any effect on viscosity,since the chains shape and the forces applied to particles are same in both directions. Also influence of AC magnetic field on viscosity of nanofluid was investigated. The results showed no change in viscosity, even with increasing the time in exposure in highest volume fraction, since changing poles frequently in alternating magnetic field doesn’t let the particles to form chains. Keywords : magneticnanofluid, Fe 3 O 4 /Ethylene glycol nanofluid,viscosity, magnetic field, capillary tube