A high-performance aromatic co-polyimide (co-PI) fiber was prepared by thermal treating the polyamic acid (PAA) precursor fiber produced through a wet spinning technique. A gradient thermal treatment protocol, which was selected based on the thermogravimetric analysis of PAA fibers, was employed for achieving the best structure and fiber performance. The structural and morphological variations of fibers at different thermal treatment stages were verified by Fourier transform infrared spectroscopy, sonic orientation detection, wide-angle X-ray diffraction and scanning electronic morphology. Thermal treatment cycloimidizes the PAA precursor into PI fibers with the concomitant development of a laminar orientation structure expanded from outer to inner layers of the fiber. The ordered chain repeat length is calculated to be 1.62 nm in the meridian direction. Mechanical measurements indicate that the synthesized co-PI fiber achieves optimum fracture strength and initial modulus up to 3.56 and 101 GPa, respectively. Thermal characterization results indicate a 5% weight loss temperature under nitrogen up to 591 A degrees C and a glass transition temperature at 340 A degrees C.