Magneto-theological nanofluids (MRNF) can be implemented in a variety of smart actuation systems including optics finishing, fluid clutches, aerospace, sealing, automotive and civil damping applications. A system based on magneto-theological nanofluids might prove itself an advanced step in the design of products with power density, accuracy and dynamic performance as key features. Additionally, for products which require controlled fluid motion by varying the viscosity, a structure based on magneto-rheological nanofluids might be an improvement in functionality and cost. This research article is mainly concerned with the preparation of paraffin oil based nanofluid containing varying percentages of Fe-Ni nano particles of (<= 15 nm) size and study of their magneto-theological behavior. The results show the big influence of particle concentration and magnetic field on the viscosity and yield stress. The optimum value of particle concentration is obtained as 10 wt.% of Fe-Ni within the range of magnetic flux density 0-1.2 T. The maximum viscosity and yield stress achieved for 10 wt.% of Fe-Ni nanoparticles are 2910 Pa s and 240 Pa respectively at 1 T magnetic field. (C) 2012 Elsevier B.V. All rights reserved.