This study focused on the inner correlation between heat treatment, microstructure and mechanical property of TNM （Ti-Al-Nb-Mo） alloy. The samples were firstly annealed at 1 270 ℃，then kept at 800 ℃ for 500， 1 000 and 2 000 hours，separately. Using X-ray diffraction（XRD），scanning electron microscope（SEM）， transmission electron microscope（TEM） and microhardness tester，the phases, microstructures and hardness of the samples bared by varied heat treatments were investigated. It was found that the boundaries of（α2+γ） colonies were boarding when the annealing duration increased；more βo and γ particles precipitated at colony boundaries and formed a cellular structure. At the same time, more nano-scaled βo particles were showed up in the single α2 lamellae，inducing α2 /βo /α2 structure and more interfaces. Therefore，the hardness of TNM alloy can be enhanced with the prolonging of annealing duration due to the bigger hardness of βo particles and α2 /βo interface which can hind dislocation movements additionally.