Poly(gylcolide-co-$varepsilon$-caprolactone)(PGCL) was synthesized by bulk ring-opening polymerization of glycolide and $varepsilon$-caprolactone using stannous octoate as a catalyst. Polymerization was conducted at a constant molar ratio of glycolidel-$varepsilon$-caprolactone (55/45). The changes of copolymer chain structure according to the reaction conditions such as reaction time, temperature and catalyst feed ratio were studied by using $^1H$-NMR spectroscopy. The effects of reaction time and catalyst feed ratio on the chain microstructure of the final PGCL copolymers were found to be significant. The average sequence length of glycolyl segments in the copolymer decreased with transesterification during polymerization, which made PGCL microstructure more random.