FTMP(friction thermo-mechanical process) is an adaptation of friction stir welding, and can be used as a generic process to modify the microstructure at selective locations. In this study, in order to analyze characteristics of surface modification of ACA4 castings by FTMP, change of rotating speed(R/S) and traveling speed(T/S) of tool were applied as conditional parameter. Analysis of microstructure, hardness, surface roughness and depth of modified zone(MZ) were searched. The best condition were obtained at R/S 600 rpm and T/S 100 mm/min. At this time, hardness was 82 HV, the surface roughness was 0.07 mm and the depth at MZ was 1.72 mm. Free defects microstructure and fine Si particles formation and strong forging effects were analyzed at MZ.