weight-average molecular weights (Mn, Mw) up to 28 500 and 78 600, respectively, and a large glass transition temperature of 260℃. Using in situ poling and temperature ramping technique, the optimal temperatures (Topt) for corona poling were obtained to get the largest second-order NLO response. The electro-optical coefficient (γ33) of a polyimide at the wavelength of 830nm was up to 22 pm/V after corona poling under its Topt, and the value remained well at elevated temperatures (retained>87% at 240℃ for more than 120h). The thermal stability of NLO polyimides was studied by the UV spectrometry after poling of the films.