Polylactic acid or polylactide (PLA) can form amorphous, semicrystalline, or highly crystalline structure. In this paper, we provide a model for semicrystalline PLA under straininduced dual-phase transformation, between mesomorphization and crystallization. In the proposed constitutive model, we use an internal state variable approach by considering initial microstructure, strain-induced dual-phase transformation, intermolecular features, and preferred network orientation-induced anisotropy. The effective contribution of the newly created phases is considered in both the elastic-viscoplastic intermolecular resistance and the viscohyperelastic network resistance using a micro-macro transition approach. We find a good agreement between the model and experimental data for the semicrystalline PLA in terms of progressive evolution of the mesomorphic/crystalline phases along with stressstrain behavior up to very large strains and over a wide range of crystallinities. The relationship between initial crystallinity, strain-induced molecular ordering, newly created phases, and mechanical response is discussed.