Cast Mg alloys were processed using friction stir processing (FSP) to attain a fine grained structure and high strength. Actually, It is a novel grain refinement method for light metal alloys and is effective to eliminate cast defects such as porosities or micro shrinkages. In this paper, the effects of FSP parameters on the mechanical properties and microstructures of as-cast magnesium AZ91 alloy are investigated. Micro hardness and Grain-growth of friction-stir-processed specimen's Nano-sized Boron Nitride (BN) particles are processed. In this present investigation, surface property of Magnesium alloy AZ91 D is modified by reinforcing Boron Nitride (BN) powder particles via FSP. By keeping volume fraction (22.38%) constant, the distribution of BN particles were examined for different reinforcing techniques, based on Hardness and Impact strength.