Methoxy poly(ethylene glycol)-b-poly(epsilon-caprolactone) (MPEG-PCL) or MPEG-b-poly(L-lactide) (MPEG-PLLA) diblock copolymers were prepared by the polymerization of CL or LA, using MPEG as an initiator in the presence of stannous octoate. MPEG-b-poly(epsilon-caprolactone-ran-L-lactide) (MPEG-PCLA) diblock copolymers with different chemical composition of PCL and PLLA were also prepared by adjusting the amount of CL and LA from MPEG in the presence of stannous octoate. In degradation study, the degradation of the MPEG-PCLA diblock copolymers mainly depends on the PCL and PLLA segments present in their structure. MPEG-PCLA, with intermediate ratio of PCL and PLLA segment, completely degraded after 14 weeks. Meanwhile, partially degraded MPEG-PCLA segments and parent MPEG segments were observed at higher PCL or PLLA segment contents. Introduction of PLLA into the PCL segments caused a lowering of the crystallinity of the diblock copolymers, thus, inducing a faster incoming of water into the copolymers. We confirmed that the diblock copolymers, with lower degree of crystallinity, have degraded more rapidly.