A new X-type polyester (4) containing nitrophenylazonitroresorcinoxy groups as NLO chromophores, which are components of the polymer backbone was prepared. Polyester 4 is soluble in common organic solvents such as N,N-dimethylformamide and dimethylsulfoxide. It shows a thermal stability up to 280 degrees C from thermogravimetric analysis with glass-transition temperature (T-g) obtained from differential scanning calorimetry near 120 degrees C. The second harmonic generation (SHG) coefficient (d(33)) of poled polymer films at the 1064nm fundamental wavelength is around 5.08 x 10(-9) esu. The dipole alignment exhibits a thermal stability even at 5 degrees C higher than T-g, and no SHG decay was observed below 125 degrees C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.