Novel functional diamine, 6-hexyloxy-naphthalen-3′,5′-diaminobenzoate (N6) containing a rigid naphthalene unit, was molecularly designed and successfully synthesized. PIs (polyimides) were obtained by copolymerization of N6, 3,3′-dimethyl-4,4′-methylenediamine (DMMDA) and 4,4′-oxydiphthalic anhydride (ODPA). The structures of the intermediates, diamines and PIs were confirmed by FT-IR and ¹H NMR spectra. All PIs obtained could be dissolved in polar aprotic solvents and low-boiling-point solvents. PI (polyimide) films attained using a casting method showed favorably high transmittance above 95% in the wave length range of 400–700 nm and could align LCs vertically before and after rubbing treatment. PI–N6 derived from N6, DMMDA and ODPA exhibited a much higher temperature at a 5% weight loss (T₅) compared with the corresponding PI–C6 from 4-hexyloxy-biphenyl-3′,5′-diaminobenzoate (C6). For PI–N6, the weight ratio of the side chains was smaller than that of PI–C6, but a much higher T₅ was attained. The results demonstrated that the introduction of a naphthalene unit into the side chain could effectively improve the thermal stability of PI without sacrificing its solubility. Moreover, the outstanding thermal stability of the PIs was explained in a preliminary manner by the imidization reaction mechanism.