β-Phase stability is a key consideration for the design of β-titanium (β-Ti) alloys and subsequent heat treatment and/or thermo-mechanical processing. The concept of the Mo equivalence (Mo-Eq), proposed by Molchanova (Phase diagrams of titanium alloys, 1965), has been commonly used as a general guideline to gauge the stability of a β-Ti alloy. A critical literature review has shown that all four existing Mo-Eq expressions deviate substantially from experimental observations and the well-established d-electron theory in predicting the β-phase stability of Ti-Nb-Zr alloys. The reasons are that existing Mo-Eq expressions either completely neglect or significantly overestimate the β-stabilizing effect of Zr. In this study, a new Mo-Eq expression, i.e., (Mo-Eq) Ti-Nb-Zr = 0.238Nb (wt.%) + 0.11Zr (wt.%) + 0.97, has been defined for Ti-Nb-Zr alloys in order to properly address the β-stabilizing effect of Zr. This new Mo-Eq expression showed proven consistency with both experimental observations and the d-electron theory in predicting the β-phase stability of various Ti-Nb-Zr alloys. With necessary modifications, the approach developed is expected to be also applicable to the assessment of the β-phase stability in other Zr-containing Ti alloys.