Nano-diamond/Ni and Ni coatings were prepared by brush plating. SEM and XRD methods were applied to investigate the morphology, nano-crystalline structure and grain size of the coatings. The results show that the grain size of the brush coatings all is in nanometer scale. The surface morphology of the composite brush coatings is a finer and denser cauliflower-like structure. Meanwhile the reasons of formation nano-crystalline structure are the high over-potential, the discontinuous grain growth resulted from the reciprocating motion and friction between the plating pen and the work piece, the heterogeneous nucleation of the nano-diamond, and the point discharge effect. The nano-diamond as the second phases decreases the grain size of the coating and consequently increase the strength of the composite coatings largely.