Strict isomorphism between butylene succinate and butylene fumarate in poly(butylene succinate-co-butylene fumarate) (PBSF) was revealed by DSC and wide-angle X-ray diffraction results. They adopt the same crystal modification, with only a little difference in crystal lattice parameters. The melting point of the copolymer increases linearly with increasing molar ratio of butylene fumarate and the melting enthalpy hardly changes, which meet the requirement of strict isomorphism. The introduction of unsaturated comonomer, fumaric acid, into PBS can enhance the total crystallization rate and the radial growth rate of spherulites. Consequently, PBF and PBSF are highly efficient polymeric nucleating agents for PBS and its copolymers. In this work, strict isomorphism provides us a new method to find polymeric nucleating agent.