A new high molecular weight polyimide based on 4,4′-oxidiphthalic anhydride (ODPA) dianhydride and 2,2′-dimethyl-4,4′-diaminobiphenyl (DMB) diamine has been synthesizedvia a one-step polymerization method. This polyimide is soluble in phenolic solvents. Films from 7 to 30 μm thick were cast from the polymer solution and show in-plane orientation on a molecular scale detected by Fourier transform infrared spectroscopy experiments. This anisotropic structure leads to anisotropic optical properties arising from two different refractive indices along the inplane and out-of-plane directions. ODPA DMB possesses high thermal and thermo-oxidative stability. The glass transition temperature has been determined to be 298 °C. Dynamic mechanical analyses show two relaxation processes appearing above room temperature: the β- and the α-relaxation processes. The α-relaxation corresponds to the glass transition while the β-relaxation is a secondary relaxation process associated with the non-cooperative subsegmental motion.