A series of diamines was polycondensed with pyromellitic di-anhydride and 3, 3′, 4, 4′-benzophenone tetracarboxylic dianhydride using a two-step method to get thin, strong, and flexible polyimide films which were characterized by the variations of the substitution at the carbon atom connecting the two phenyl rings of the diamine moiety. The poly(amic acid) prepolymers obtained in the first step had inherent viscosities ranging from 0.9 to 1.60 and they were converted to polyimides by thermal cyclodehydration. All the polycondensation reactions were carried out in dimethyl acetamide under identical conditions. Thus 16 different polyimide polymers were synthesized and characterized by elemental analysis, IR spectroscopy, and viscosity measurements. Some other important properties, such as solubility, thermal stability, and electrical behavior, were also investigated. The alkyl chain length affected the flexibility of the resultant polyimide films greatly and the electrical properties slightly; however, thermal stability was decreased subsequently.