The strengths of the a precipitate and the b matrix were evaluated by nanohardness in the Ti215Mo21Fe and Ti215Mo23Al alloys and compared to those of the Ti215Mo alloy. The a phases with similar size (a long axis of a few micrometres and a short axis of a few hundred nanometres), distribution and volume fraction were obtained in three alloys by adjusting the aging temperature. Analyses by SEM-EDS confirmed that Fe and Mo were enriched in the b phase and depleted in the a phase, while Al was enriched in the a phase and depleted in the b phase. Tensile tests were carried out, and the tensile strength was shown to be higher in the Ti215Mo21Fe and Ti215Mo23Al alloys than in the Ti215Mo alloy. The nanohardness measurements indicated that the a phase was softer than the b phase in both Ti215Mo21Fe and Ti215Mo alloys, while it was harder in the Ti215Mo23Al alloy. The increased tensile strength was mainly caused by the strength of the Fe enriched b phase in the Ti215Mo21Fe alloy and by the strength of the Al enriched a phase in the Ti215Mo23Al alloy.