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Single electrospun copolyimide nanofiber and aligned co-PI nanofiber belts with high strength and high modulus were prepared from copolyamic acid containing pyrimidine units. The chemical structures of copolyamic acid and copolyimide were characterized by 1H-NMR and FT-IR spectra. The morphology of the electrospun nanofibers was investigated by SEM and AFM. The mechanical properties of the single electrospun nanofiber and aligned nanofiber belts were measured by the single fiber micro-tensile test and the uniaxial tensile test, respectively. The thermal properties of the co-PI nanofiber belts were evaluated on TGA and DMA machines. The results indicated that the incorporation of rigid pyrimidine units, the molar ratio of pyrimidine/4,4′-oxydianiline and the imidization temperature played an important role in the mechanical properties of electrospun copolyamic acid and copolyimide nanofibers while the thermal properties were influenced by the molar ratio of rigid and soft components in the copolyimide main chains. [ABSTRACT FROM AUTHOR]