3,4-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-4'-nitrostilbene dianhydride was prepared and reacted with 4,4'-(hexafluoroisopropylidene)dianiline to yield a novel Y-type polyimide containing the 3,4-dioxynitrostilbenyl group as an NLO-chromophore, which constituted part of the polymer backbone. The resulting polyimide was soluble in polar solvents such as acetone and N,N-dimethylformamide. The polymer exhibited good thermal stability up to 370 ℃ in the thermogravimetric analysis. The glass-transition temperature (Tg) obtained from the differential scanning calorimetry thermogram was near to 153 ℃. The second harmonic generation (SHG) coefficient (d33) of the poled polymer film at the fundamental wavelength of 1064 cm-1 was around 2.15 × 10-8 esu (9.01 pm/V). The dipole alignment exhibited exceptionally high thermal stability even at a temperature 30 ℃ above the Tg, and there was no SHG decay below 180 oC because of the partial main chain character of the polymer structure.