For pure Mo and dilute Mo-Ti Alloys carburized in mixed gases of argon and carbon monoxide, microstructural observation through a transmission electron microscope, X-ray diffraction analysis and hardness measurements were carried out. Static three-point bend tests were also performed at temperatures from 77 to 298 K. In the case of Mo-Ti alloys, hardening due to the internal carburizing was recognized, whereas no hardening was observed in the case of pure Mo. The hardness of the internal carburizing layer exhibited Hvsimilar to300 for the specimen carburized at 1673 K for 16 h, gradually decreasing as the depth from specimen surface increased. The rate of hardening was strongly dependent on the carbon monoxide concentration. X-ray diffraction analysis for carburized specimens revealed that brittle alpha-Mo2C layer was hardly formed on the specimen surface in the case of low carbon monoxide concentration. These results suggest that gas carburization using mixed gases of argon and carbon monoxide is effective to the strengthening of Mo-Ti alloys.