Unsaturated groups were introduced into the main chains of poly(butylene succinate) (PBS) by the condensation polymerization of 1,4-butanediol with succinic acid and maleic acid (MA). The resulting aliphatic polyesters were subjected to chain extension via the unsaturated groups with benzoyl peroxide (BPO), BPO/ethylene glycol dimethacrylate (DF), or BPO/triallyl cyanurate (TF). During the condensation polymerization, some of the cis-structured maleate was isomerized into the trans-structured fumarate. The trans-structured fumarate participated in the chain-extension reaction with BPO more than the cis-structured maleate. However, the trans/cis ratio remained practically unchanged when bridging molecules such as DF and TF were used along with BPO. Chain extension of PBS containing 5.7 mol % MA units (PBSM57) resulted in gel formation. Chain extension with BPO/TF made more gels in PBSM57 than chain extension with the other crosslinking agents. Chain extension increased the glass-transition temperature, decreased the melting temperature and crystallinity, and improved mechanical properties such as elongation and tensile strength. The results of the modified Sturm tests showed that the biodegradability of the unsaturated aliphatic polyesters decreased greatly because of the chain extension.