Friction stir processing is an energy efficient solid-state processing route for the fabrication of aluminium matrix surface composites with better interfacial bonding between the matrix and reinforcement particles. Aluminium matrix surface composites with metallic Mo reinforcement fabricated by FSP show improved surface properties such as hardness, and corrosion resistance compared to the unprocessed base material. The reinforcement distribution and the corresponding hardness behavior of surface composites fabricated by single and double pass processing are analyzed. The particle size distribution before and after the processing is measured to understand the effect of mechanical shearing of reinforcement particles. The grain refinement due to the plastic deformation during FSP and the distribution of finer Mo particles enhanced the surface hardness of the composites. The surface composites exhibited two-fold increase in the surface hardness compared to the unprocessed base material. The study on metallic Mo reinforced aluminium matrix composites using FSP groove method is yet to be studied in detail. The effect of processing passes on the particle distribution and surface hardness is investigated in this research work. Knowledge about the effect of processing and Mo particles on the surface properties will enable the development of composite materials with enhanced surface properties having better interfacial bonding between the matrix and the reinforcement particles.