In this work, we investigate the strain hardening behavior of the metastable β Ti-12Mo-5Zr (wt. %) alloy combined with TWIP/TRIP effects (TRIP for Transformation Induced Plasticity and TWIP for Twinning Induced Plasticity). The evolution of the strain hardening exponent is analyzed by classical Hollomon theory. The three stages of strain-hardening behavior are divided, labeled as Stage ?, Stage ?? and Stage ???, respectively. Stage I corresponds to the transition from the elastic to plastic regimes (ε≈0~0.02). Stage II relates to a continuously increase of the strain-hardening rate and the hardening exponent (ε≈0.02~0.15). Then, the strain-hardening rate decreases and the hardening exponent reaches stable value 0.34 in Stage III (ε≈0.15~0.34). The evolution of microstructure within three stages is observed by employing techniques of optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). Based on the experimental results, the governing mechanisms for deformation at different stages and strain-hardening behavior of Ti-12Mo-5Zr alloy are discussed.