Abstract: This paper is to investigate the mechanical properties and deformation mechanisms at low temperatures of Ti-5AI-2Mo-3Zr alloy. The results showed that quenched samples kept theit high plasticity until -75℃.while the tempered samples were with very low elongation.This means that alloying zirconium is beneficial to low temperature plasticity only when it is at solution state, but not at precipitation state.The deformation mechanism changed from slip to twin during testing temperature lowering.TEM observation showed a special morphology of double "triple twins",may indicating that the crystals rotated from each other a rather large angle during low temperature deformation.