A series of co-polyimides (co-PIs) containing pyrimidine rings were synthesized by reacting the self-synthesized diamine 2,5-bis(4-aminophenyl)-pyrimidine (PRM) with commercial 4,4'-diaminodiphenyl ether (ODA) and aromatic 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA). A series of co-PI fibers were produced using a two-step wet-spinning method, including extrusion, coagulation, washing, drying, thermal imidization and hot-drawing processes. Orientation structures of the precursor co-PAA fibers formed in the wet-spinning process, and their effects on the thermomechanical properties of the as-spun co-PI fibers were discussed. An interesting phenomenon has been found that glass transition temperatures (T(g)s) of as-spun co-PI fibers decreased with increasing orientation factors, which was explained based on some previous works. Two-dimensional wide-angle X-ray diffraction spectra of the final co-PI fibers showed that samples with higher PRM contents show highly oriented and well-ordered crystalline structures, which account for the enhanced mechanical properties when incorporating PRM moieties. The co-PI fiber with the PRM/ODA molar ratio of 6/4 showed an optimum tensile strength and modulus of 3.1 and 90 GPa, respectively, showing a great potential in light advanced composites.