A series of polyimide (PI) fibers synthesized from pyromellitic dianhydride (PMDA), 4,4'-oxydianiline (ODA), and p-phenylenediamine (p-PDA) were prepared by a two-step wet-spinning process. The prepared PI fibers were then carbonized with the increasing temperatures up to 1500 A degrees C under a high-purity nitrogen atmosphere. The effects of ODA/p-PDA molar ratios on the chemical structure, microstructure, chain orientation, and structural evolutions of the PI fibers were systematically investigated. The elemental composition, morphology, and aggregation structure after carbonization were also analyzed. The results showed that different chemical compositions have greatly influenced the aggregation structures of the resulting PI and carbon fibers. The PI fibers showed increased crystallinity and orientation degree with the decreased ODA moieties, while the corresponding PI-based carbon fibers (CFs) exhibited perfect graphitic structures. The CFs derived from PMDA/p-PDA PI backbone with flat chains exhibited a well-defined graphite structure with d (002) of 0.349 nm and I (D)/I (G) of 1.442. In addition, the carbon yield of the prepared PI-based carbon fibers reached more than 95 %.