Fe3O4　nanoparticles　were　prepared　by　co-precipitation　and　coated　by　sodium　dodecyl　benzene　sulfonate　(SDBS)　to　obtain　water-based　magnetic　fluid.　The　viscosity　of　the　magnetic　fluid　was　measured　using　an　Ubbelohde　viscometer.　The　effects　of　magnetic　particles　volume　fraction,　surfactant　mass　fraction　and　temperature　on　the　viscosity　were　studied.　Experimental　results　showed　that　the　magnetic　fluid　with　low　magnetic　particle　volume　fraction　behaved　as　a　Newtonian　fluid　and　the　viscosity　of　the　magnetic　fluid　increased　with　an　increase　of　the　suspended　magnetic　particles　volume　fraction.　The　experimental　data　was　compared　with　the　results　of　a　theoretically　derived　equation.　The　viscosity　of　the　magnetic　fluid　also　increased　with　an　increase　in　surfactant　mass　portion,　while　it　decreased　with　increasing　temperature.　Moreover,　the　viscosity　increased　with　increasing　the　magnetic　field　intensity.　Increasing　the　temperature　and　the　surfactant　mass　fraction　weakened　the　influence　of　the　magnetic　field　on　the　viscosity　of　the　magnetic　fluid.　©　2013　Elsevier　B.V.