BTDA-TDI/MDI (P84, synthesized by the condensation of 2,4-diisocyanato-1-methylbenzene and 1,1'-methylenebis(4-isocyanatobenzene) with 5,5'-carbonylbis(l,3-isobenzofurandione)) co-polyimide fibers were prepared by wet-spinning. The basic spinning conditions were found from the studies of dope viscosity, ternary phase diagrams, coagulation value, and precipitation value. The effect of the coagulation bath composition on the morphology of as-spun fibers was investigated and a theoretical approach was used to understand the coagulation phenomena. Scanning electron microscopy (SEM) showed that the cross-sectional shape of the fiber deviated more from an ellipse shape with the increasement of N-methyl-2-pyrrdidinone (NMP) content. The surface and cross section morphology of the as-spun fibers was also analyzed by the rate of diffusion and phase separation. The as-spun fibers were treated in heating tubes without drawing at different temperatures. The gravimetric analysis spectra showed that the BTDA-TDI/MDI co-polyimide fibers, which had been heat treated at 350 and 400℃ , possessed better thermal properties than the as-spun fibers, a large weight loss was observed only above 550 ℃. Heat treatment of the fibers resulted in relatively high tensile strength and modulus. The fibers spun in Bath C (70/30, NMP/water, wt/wt) and Bath D (80/20, NMP/water, wt/wt) showed better thermal properties and higher tensile strength.