Semicrystalline polymers usually undergo multilevel microstructural evolutions under high-temperature annealing and stretching deformation; this is essential to tailor the physical properties of polymer products in industrial processing. Here, we choose poly(p-dioxanone) (PPDO), a typical biodegradable, biocompatible, and bioresorbable polymer, as a model semicrystalline polymer and investigated its polymorphic structural transition and crystalline lamellar evolution under high-temperature annealing and stretching. High-temperature annealing caused the alpha'-to-alpha phase transition of PPDO, accompanied by the improvement of crystallinity (X-c) and thickening of crystalline lamellae. Tensile strength and Young's modulus of PPDO increased but the breaking strain decreased as the annealing temperature increased. Stretch-induced phase transition of PPDO depended strongly on the initial structure and stretching temperature (T-s). The a form PPDO transformed into its alpha' counterpart during stretching at low T-s. This phase transition was irreversible and did not retain the alpha form with the release of stress. However, no phase transition took place for the alpha-form PPDO stretched at high T-s (>= 40 degrees C). Original lamellae of alpha-form PPDO changed into the fibrillar lamellae during stretching via the melt-recrystallization mechanism.