Low modulus β Ti alloys are attractive for biomedical application. This work examines the mechanical properties of Ti-Cr-Sn-Zr system alloys. The Young's modulus of the alloy varies with the composition, which variation is caused from the competition between the meta-stable β phase and ω phase. The Young's modulus of the alloy decreases very much owing to the addition of an alloy element that depresses ω phase, which trend is similar to Ti-Nb system alloys. This work focuses on the effect of the varying alloy composition on the microstructure, the Young's modulus, the deformation mechanism and the deformability. The deformation modes of the Ti-Cr-Sn-Zr alloy, which are the mechanical twinning, the deformation by slip and the deformation-induced transformation, change by the composition of the alloy. We discuss the effect of the transition of the deformation modes on the mechanical properties.