A newly synthetic method has been developed for the preparation of block copoly(urethaneimide) (PUI). The copolymer was yielded from the direct reaction of the prepolyurethane terminated with isocyanate and the prepolyimide terminated with anhydride. The latter is prepared by reacting an excess amount of pyromellic dianhydride (PMDA) with bis[4-(3-aminophenoxy)phenyl] sulfone (BAPS) in one-step polymerization. The structure was confirmed by infrared (IR) spectra. Other characterizations were evaluated. Among them were solubility, viscosity, mechanical properties, and thermal properties.