Two organo-soluble aromatic polyimides have been synthesized by introducing pendant groups of trifluromethyl or methyl onto the 2- and 2′-positions of a biphenyl diamine and reacting each of them with biphenyltetracarboxylic dianhydride (BPDA): 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (PFMB) or 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB). These two polyimides, BPDA-PFMB and BPDA-DMB, are prepared via a one-step poly condensation method, and they can be fabricated into fibers, films, and other application forms. Our research shows that in fiber applications, these two fibers exhibit excellent mechanical properties and outstanding thermal and thermooxidative stability. Their long-term mechanical tensile performance at high temperatures is found to be critically associated with the type of side pendant groups at the 2- and 2′-positions of the diamines. High-resolution pyrolysis-gas chromatography/mass spectrometry (PyGC-MS) results show that the pyrorgams of these two polyimides possess more than 16 pyrolyzates. Based on the compositions and distributions of the pyrolyzates at different temperatures, the thermal degradation mechanisms for both of the polyimides are found to be the same.