The crystallization kinetics and morphology of poly(butylene succinate-co-ethylene succinate) (PBES), a biodegradable copolyester with slow crystallization rate, were systematically investigated in this work using poly(butylene fumarate) (PBF) as a novel polymeric nucleating agent. The effects of different PBF contents on the crystallization process of PBES were studied under both the nonisothermal and isothermal melt crystallization conditions. The nonisothermal melt crystallization peak temperature values of PBES were increased by about 18 and 23 째C, respectively, in the presence of only 1 and 2 wt% PBF. Under the isothermal melt crystallization conditions, PBF also accelerated the crystallization process of PBES. When PBES was isothermally crystallized at 90 째C, the crystallization half-time values were obviously reduced from 21.1 min in the absence of nucleating agent to 13.6 and 4.9 min when the PBF contents were 1 and 2 wt%, respectively. The spherulitic morphology study revealed that PBF acted as an efficient nucleating agent for the crystallization of PBES, as the spherulites size became significantly smaller with increasing the PBF content. The crystal structure of PBES remained unmodified in the presence of PBF. The nucleating mechanism was also discussed.