Titanium and its alloys have been extensively used in numerous application fields. However, both their low ductility and their lack of work-hardening when compared with steels or Co-Cr alloys, limit their use in advanced applications where superior combinations of strength and ductility are required. Therefore, inspired from the superior mechanical properties of TRIP/TWIP steels (TRIP for Transformation Induced Plasticity and TWIP for Twinning Induced Plasticity), we proposed a new type of metastable β Ti-alloys showing combined TRIP/TWIP effects through controlling the β stability of Ti-alloys. The alloys were designed based on the “d-electron alloy design method” and controlling of electron/atom ratio (e/a). The results show that so-designed alloys display excellent combination of high strength (ultimate tensile strength ≈1020MPa), high ductility (uniform elongation 43%) and improved work-hardening behavior. The detailed microstructural analysis indicates that the superior performances arise from the synergic effects between α? phase transformation induced plasticity (TRIP) and {332}<113> twinning induced plasticity (TWIP).