The microstructure of Ti-6Al-4V alloy treated with Fine Particle Peening (FPP) was studied. The influence of different peening times, particle supply rates, and particle sizes on the microstructure was especially investigated. FPP treatment using steel particles created a modified microstructure with a lamellar feature beside the treated surface. The modified structure showed higher hardness comparing to the interior region and it was accompanied by Fe elements transferred from particles. Energy Dispersive X-ray Spectroscopy (EDS) analyses revealed that the amount of transferring Fe elements was increased as peening times and/or particle supply rates were increased. Fe transfer was also accelerated by FPP treatment using smaller particles. The reasons for this behavior were as follows: (1) plastic deformation induced by collisions of particles was effectively accumulated with an increase of peening time and particle supply rate, and (2) plastic deformation induced by collisions of particles became more severe as particle size decreased. The accumulation and severity of plastic deformation strongly affected the formation of the modified microstructure enriched with transferred elements.