The effect of Ni nanoparticles concentration on the rheological and magneto-rheological properties of Ni ferrofluid (FF) in liquid paraffin base were investigated. The FFs were prepared using a high energy milling. A high-energy mill was used to prepare these Ni based ferrofluids. By measuring the peak broadening of the (111) crystal plane for Ni from the X-ray diffraction spectrum of the dry nanoparticles, the mean crystallite size was 38 nm. Regardless of nanoparticle concentration, the fluids were found to have non-Newtonian shear thinning behavior. At a constant shear rate, the viscosity of the nanofluids increased as the Ni nanoparticle concentration was increased. This experimental data was then compared to existing models; none of which adequately described the measured rheological properties of the suspensions. When an external magnetic field was applied at a constant shear rate, the viscosity increased to a plateau, then decreased as the field strength was increased further. It is thought that as the magnetic field strength is increases, larger flocculated nanoparticle structures form within the fluid. Although there are larger flocculants, there are fewer of them resulting in a decrease in the viscosity of the suspension. (C) 2015 Elsevier Ltd. All rights reserved.