The objective of this research is to investigate the molecular weight, degradation mechanism, and crystalline morphology of polyethylene glycol/poly (L-lactide) (PEG/PLLA) copolymers during hydrolytic degradation. PEG/PLLA copolymers were prepared from cyclic L-lactide and polyethylene glycol with molecular weights ranging from 1000 to 6000 Da. The structural analysis was carried out by GPC, DSC, FTIR, C-13-NMR, and H-1-NMR. Gel permeation chromatograms also indicate that the hydrolysis causes the change of mass distribution from a unimodal to a bimodal form. An exothermic recrystallization peak and its shoulder portion at the lower temperature range during melting appears immediately following the hydrolytic degradation. This indicates the heterogeneity of the crystals. The data of NMR and FTIR shows that during the initial period (0-200 h) of hydrolysis, there appears to be a formation of hydroxyl end groups connected to PEG blocks and carboxyl end groups connected to polylactide blocks. Due to the hydrophilic ethylene oxide segment in PEG/PLLA copolymers, the rate of hydrolysis is much faster during the first 200 h relative to longer hydrolysis time. It is therefore concluded that the chain scission during the initial period occurs at the ester linkage connecting PEG and PLLA blocks, in addition to ester groups within the PLLA blocks.