Novel phosphorus-containing copolyester nanocomposites were synthesized by in situ polymerization with 2-carboxyethyl(phenylphosphinic) acid (CEPPA) and nano-ZnCO3. The flame retardancy and static and dynamic mechanical properties of poly(ethylene terephthalate) (PET)/nano-ZnCO3 composites and phosphorus-containing copolyester/nano-ZnCO3 composites were evaluated with limiting oxygen index measurements, vertical burning testing (UL-94), a universal tensile machine, and a dynamic mechanical analysis thermal analyzer. The phosphorus-containing copolyester nanocomposites had higher limiting oxygen indices (ca. 32%) and a V0 rating according to the UL-94 test; this indicated that nano-ZnCO3 and CEPPA greatly improved the flame retardancy of PET. The static mechanical test results showed that the breaking strength, modulus, and yield stress of the composites tended to increase with increasing nano-ZnCO3 content; when 3 wt % nano-ZnCO3 was added to PET and the phosphorus-containing copolyester, the breaking strength of the composites was higher than that of pure PET. Dynamic mechanical analysis indicated that the dynamic storage modulus and loss modulus of the PET composites increased markedly in comparison with those of pure PET. However, the glass-transition temperatures associated with the peaks of the storage modulus, mechanical loss factor, and loss modulus significantly decreased with the addition of ZnCO3 and CEPPA. The morphologies of the composites were also investigated with scanning electron microscopy, which revealed that nano-ZnCO3 was dispersed homogeneously in the PET and copolyester matrix without the formation of large aggregates. In addition, the interfacial adhesion of nano-ZnCO3 and the matrix was perfect, and this might have significantly affected the mechanical properties of the composites. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 3131-3136, 2011