Poly(butylene succinate-co-diethylene glycol succinate) (P(BS-co-DEGS)) random copolymer was synthesized and characterized in a previous paper [Ind. Eng. Chem. Res. 2012, 51, 12258-12265]. In this paper, we focus on the isothermal crystallization behaviors of P(BS-co-DEGS). The isothermal crystallization kinetics, spherulitic morphology, and growth kinetics of P(BS-co-DEGS) were investigated by differential scanning calorimetry and polarized optical microscopy and compared with those of neat poly(butylene succinate) (PBS). The results suggest that the crystallization rate of P(BS-co-DEGS) was much slower than that of neat PBS and decreased with increase of DEGS content, while the crystallization mechanism remain unchanged. P(BS-co-DEGS) showed banded morphology, and the band spacing decreased with increase of DEGS content at a given supercooling. The spherulitic growth rate of P(BS-co-DEGS) decreased with increase in DEGS content. A transition from crystallization regime II to crystallization regime III occurred for all samples, and the transition shifted to lower temperatures with increase in DEGS content.