Novel X-type polyester 5 containing 5-nitro-2,4-dioxybenzylidenecyanoacetate groups as nonlinear optical (NLO) chromophores, which constitute parts of the polymer backbone, was prepared and characterized. Polyester 5 is soluble in common organic solvents such as N,N-dimethylformamide and acetone. Polymer 5 shows a thermal stability up to 280 A degrees C from thermogravimetric analysis with glass-transition temperature obtained from differential scanning calorimetry of near 116 A degrees C. The second harmonic generation (SHG) coefficient (d (33)) of poled polymer film at the 1064 nm fundamental wavelength is 1.78 pm V-1. The dipole alignment exhibits a thermal stability even at 4 A degrees C higher than glass-transition temperature (T (g)), and no significant SHG decay is observed below 120 A degrees C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.