Carbonization of Si (100) using subsurface implantation (subplantation) of low-energy mass-separated carbon ion beams was undertaken and its reaction temperature and mechanism were compared with conventional methods using thermal gas species. Si (100) surfaces were directly irradiated by mass-separated low-energy (20-700 eV) carbon ion beams, e.g., C-, C-2(-), C+ and CH3+ ions in the temperature range of 440-700 degreesC. RBEED measurements were performed for in-situ characterization of the Si surfaces during ion irradiation. All the RHEED images indicated that carbonization has occurred at 500-700 degreesC which are relatively low compared to conventional methods. The estimated activation energies of carbonization using ion beams were lower than those using gas species. It is likely that the difference in the carbonization temperature stems from the different reaction mechanisms each other.