The present work investigated the tribological properties of zinc oxide nano friction modifiers (FM) based castor oil in different concentrations between steel surfaces. The nano lubricants were prepared by dispersing zinc oxide nano FMs of varying weight percentage. The anti-wear and extreme pressure tests were conducted using a four-ball test rig. The surfaces of the tested balls were characterized using scanning electron microscope, energy dispersive system and three dimensional profilometer. After the tribo tests it was observed that both anti-wear and extreme pressure properties of the castor oil samples enhanced with the addition of ZnO nano friction modifiers up to a certain concentration of zinc oxide (0.1%) nano friction modifiers, beyond which not much improvement in the coefficient of friction was noticed but wear rate increased. The formation of tribo film due to the adsorption and adherence of nano friction modifiers on the surfaces were responsible in reducing the wear properties. The results show that castor oil can be a good choice as a green lubricant for the replacement of commercial mineral oils, with optimal additivation. (C) 2017 Elsevier B.V. All rights reserved.