Fatigue properties and deformation mechanism of Ti2.5Cu alloy were studied after solution and two-step aging at 293 K and 77 K. The results show that the fatigue plastic index of Ti2.5Cu alloy at 293 K and 77 K were –0.68 and –0.5, respectively. At 293 K dislocation slip mechanism was dominant. However, at 77 K twinning shear was rather active and various types of twins were found. Meanwhile, needle-like Ti2Cu strengthening phases hindered movement of dislocation and brought about obvious cyclic hardening, but they had little effect on the twinning process.