Based on previous research of Ti40 alloy two typical heat treatments were designed, e.g. 1#, 600 ℃, 4 h AC；2#, 850 ℃, 1 h WQ +550 ℃, 6 h AC. The microstructures , mechanical properties at room-temperature, thermal stability and creep properties of the alloy treated by the two heat treatments were tested. The result show that the difference of their microstructures is mainly in the respect of their grain boundary, e.g. the grain boundary of the alloy by 1# heat treatment is unsmooth and ragged and the ones by 2# heat treatment are smooth and straight. The mechanical properties indicated the difference of the microstructures leads a little effect on properties at room-temperature. Relatively speaking, the microstructure with the unsmooth grain boundary after 1# heat treatment brings good thermal stability and creep properties at lower temperature, and the microstructure with the straight boundary after 2# heat treatment brings good creep properties at higher temperature. The mechanism of the effect of different grain boundaries on thermal stability and creep properties was also discussed in this paper