Poly(lactic acid-b-p-dioxanone) block copolymers (P(LA-b-PDO)) were synthesized by ring-opening polymerization of p-dioxanone using poly(lactic acid) (PLA) with different molecular weights as macroinitiators in N-2 atmosphere. The copolymers were characterized by H-1-NMR spectra. The thermal and crystalline behaviors and thermal stability of these copolymers were investigated by DSC, TGA and WAXD. The results indicated that the contents of each segment and the intrinsic viscosities affected the properties of copolymers obviously.