Nonlinear optical (NLO) copolyimides exhibiting high glass transition temperatures (T-g) and high thermal stabilities were synthesized to achieve long-term stability of optical nonlinearity at elevated temperatures. The relationship between the chromophore loading level and the physical properties of the copolymers was studied. It was found that decreasing the chromophore loading level increased the glass transition temperature more than 40 degrees C. As a result, the stability of the electric field induced dipole orientation of the nonlinear optical chromophores was enhanced. The second harmonic generation (SHG) signal maintained 85% of its initial value after 700 h at 180 degrees C. Sizable second-order nonlinear optical coefficients and electro-optic coefficients were obtained.