An energy-saving mechanical trimming technology for hot stamped ultra high strength steel sheets is suggested in this study, in which the sheet thickness on the desired trim line is reduced by hot half-trimming at the same time as hot stamping, and the half-trimmed part is subsequently fully trimmed by mechanical trimming. Effectiveness of the proposed process has been investigated by FE-simulation and experiment. In this process, the hot half-trimming depth in hot stamping and the clearance between the punch and die in mechanical trimming were considered to be the main process parameters determining trimmed surface quality, trimming load, and tool wear. A high-quality trimmed surface, as compared with that produced by conventional cold trimming, can be achieved with a large hot half-trimming depth in hot stamping and small clearance in mechanical trimming. It was also shown that production time and cost can be reduced considerably by mechanical trimming using hot half-trimming, with low probability of tool chipping and reduced tool wear, as compared to conventional cold trimming and laser cutting processes. Therefore, mechanical trimming using hot half-trimming as proposed in this study can replace the high energy- and production time-consuming laser cutting process in the mass manufacturing of automotive parts.