The Lifshitz–Cahn–Allen kinetic theory of second-order phase transition has been successfully applied to the case of microstructure development of Zr alloy subjected to different cooling rates under the allotropic β→α transformation. In particular, the dependence of α-Zr lamella width on quenching rate observed in the Widmanstätten structure of the material is captured. The effect of the interstitial element oxygen, acting as a driving conveyor for transformation, is considered as an external field within the Ginzburg-Landau free energy framework.