New copolymers (PMBz) from the benzoxazole-based vinyl monomer and methylmethacrylate as a comonomer, were synthesized for an improvement of thermal stability of second-order nonlinear optical activity. The thermal stability of the chromophores in PMBz copolymers monitored by IR and UV-visible spectroscopy was much better than in PMSt copolymer bearing general pendant stilbene units. The electro-optic coefficient r(33) of the poled PMBz film exhibited as large as 45 pm/V at 632 nm of He-Ne laser. Thermal relaxation behavior of the poled PMBz films at 100 degrees C showed that the PMBz having more rigid benzoxazole structure had better temporal stability of r(33) compared to the general stilbene structure.