To clarify the differences of mechanical properties and deformation behaviors of a Ti-5Al-5Mo-1Fe-1Cr (wt.%) alloy with duplex and single-phase microstructures, the tensile properties and deformation modes of the alloy with these two different microstructures were studied systematically. The results show that the sample with the duplex microstructure has a tensile yield strength of 886 MPa, an ultimate tensile strength of 1075 MPa and an elongation of 21.5%. The sample with the single β phase has a relatively lower strength and a similar elongation. The sample with the duplex microstructure has a ductile fracture mode and deforms via dislocation slipping in the β matrix and spherical or plate-like α phases. The slip systems are in β matrix, and and in α phase. In contrast, the single-phase sample has a quasi-cleavage fracture mode and deforms via dislocation slipping and stress- induced α″ martensite transformation, and the slip systems are and .