Novel X-type polyester 5 containing 4-(2'-carbomethoxy-2'-cyano)vinyl-6-nitroresorcinoxy group as nonlinear optical (NLO) chromophore, which constitutes parts of the polymer backbone, was prepared and characterized. Polyester 5 is soluble in common organic solvents such as N,N-dimethylformamide and acetone. Polyester 5 shows a thermal stability up to 280 degrees C from thermogravimetric analysis with glass-transition temperature obtained from differential scanning calorimetry of near 116 degrees C. The second harmonic generation (SHG) coefficient (d(33)) of poled polymer film at the 1064nm fundamental wavelength is 4.25 x 10(-9) esu. The dipole alignment exhibits a thermal stability even at 4 degrees C higher than glass-transition temperature (T-g), and no significant SHG decay is observed below 120 degrees C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.