There are few previous researches on the effect of external field on thermal and rheological characteristics of nanofluids and nanomagnetic fluids. Increase in nanofluid viscosity compared to base fluid has negative effects on heat transfer and traort properties of nanofluids and therefore, trying to improve these properties by making excess motions in nanofluid would be of desire. Applying external field (magnetic or electric) on, specially, magnetic nanofluid would enhance these properties. In this study, viscosity changes of magnetic nanofluid (Fe 3 O 4 / Ethylene glycol) in the presence of external field (electric and magnetic field) were investigated experimentally. The effects of volume fraction of the suspended nanoparticles, the external electric and magnetic field intensity, the type of electric field (AC or DC), and temperature on the viscosity of the nanofluids were analyzed. The capillary tube viscometer was used for measuring the viscosity and Brookfield rotational viscometer was used characterizing the rheological behavior of base fluid and nanofluid. Fe 3 O 4 nanoparticles were dispersed in pure Ethylene glycol as base fluid in 0.01, 0.015, 0.02, 0.035 and 0.05% volume fraction concentrations. The viscosity of the magnetic nanofluids was measured in either the absence or the presence of the external fields. The accuracy of capillary viscometer was examined by comparing of experimental data in different temperature to reference data. The obtained results from rotational viscometer indicated that the base fluid and nanofluid have Newtonian behavior in the experimental conditions. Addition of nanoparticles in to the base fluid decreases its viscosity. The viscosity reduces from 6.3 to 10.1% with increasing nanoparticle concentration from 0.01 to 0.05Vol%. Changes of base fluid viscosity and nanofluids viscosity were investigated under different intensities of DC and AC electric fields. Results showed that the viscosity decreases with application of DC and AC electric fields. The effect of DC electric field on decreasing the viscosity of the magnetic nanofluids is greater than AC electric field, but about the base fluid, the effect of AC electric field is greater. Also, the effect of magnetic field perpendicular to the direction of flow on the viscosity of magnetic nanofluids was investigated. The magnetic field was applied by two strong magnets on both sides of the capillary tube and magnetic field intensity was controlled by changing in the distance of magnet segments. Increase in the magnetic field intensity cause viscosity reduction. Also, the viscosity decreases with exposure time increase of nanofluids in the magnetic field. No effect was observed on viscosity of base fluid under magnetic field. Increase in concentration of nanoparticles leads to increase in the effect of electric and magnetic fields. Key words: Viscosity, nanofluid, ethylene glycol, Fe 3 O 4 nanoparticles, capillary tube viscometer, electric field, magnetic field