Molybdenum (Mo), possessing higher hardness compared to aluminum (Al), is used for the surface hardness improvement in Al 1050 alloy. Al-Mo surface composites are developed using friction stir processing with different tool shoulder diameters. Single groove composite (SGC) and double groove composite (DGC) samples are processed with 22-, 20-, and 18-mm shoulder diameter tools to study the effect of tool shoulder diameter and Mo content on the surface hardness of Al-Mo surface composites. Microhardness increased from 26 to 51 Hv on the top surface and 52 Hv on the cross section surface, in 18 DGC. The surface hardness of the composite samples increased with decrease in tool shoulder diameter due to the combined effect of grain refinement and Mo reinforcement particles. Unprocessed Al 1050 base alloy possessed an average grain size of ~ 99 µm. The grain size in the processed samples reduced with decrease in tool shoulder diameter. Surface composites processed with 22-, 20-, and 18-mm tools exhibited an average grain size of ~ 23, ~ 22, and ~ 21 µm, respectively. The amount of Mo particles incorporated in the composite material increased with decrease in tool shoulder diameter. Microstructural analysis confirmed that the FSP groove method incorporated about ~ 8 wt pct Mo in 22 SGC and ~ 12 wt pct in 22 DGC, respectively. The amount of Mo particles in 20 SGC and 20 DGC is ~ 17 wt pct and ~ 22 wt pct, respectively, whereas 18 SGC and 18 DGC possessed ~ 27 wt pct and ~ 31 wt pct Mo particles. Mechanical shearing through severe plastic deformation during the friction stir processing reduced the average Mo particle size in the surface composites. Molybdenum is distributed in its elemental form in the Al-Mo surface composites without any intermetallic formation.