Thermal stability of polymer waveguide devices is a key issue for their commercialization. Thermal stability of device is decided by thermal stability of waveguide layer (polymer). The thermally stable modulators using side-chain polyurethane and cross-linked polyurethane with same electro-optic (EO) coefficient property at room temperature have been investigated. The thermal stability is studied by measuring the change of EO coefficient of polymer modulator during ascending the curing temperature rise. The experimental results show that the cross-linked polyurethane presents a remarkably controlled significant improvement of thermal stability.