Compositional tuning of phase stability of the body-centered cubic (BCC) matrix is an effective way to improve mechanical properties of refractory medium/high entropy alloys, which are generally brittle or lack of strain hardening capability. Here, reducing Zr content in Ti(80-x)ZrxHf10Nb10 (at.%) alloys was demonstrated to destabilize the BCC phase and trigger the BCC to hexagonal close-packed (HCP) phase transformation. Two metastable Ti50Zr30Hf10Nb10 and Ti60Zr20Hf10Nb10 medium entropy alloys were designed, which exhibit superior strength-ductility combination and evident strain hardening due to the transformation induced plasticity effect.