We developed an apparatus to measure the thermal diffusivity of a nonlinear optical (NLO) polymer thin film where the measurement was based on the photoacoustic method. The validity of the measured data was verified by comparison with published ones for Ti and Pt metallic foils. The thermal diffusivity of a polymer thin film, polymethyl methacrylate (PMMA) with Disperse Red 1 (DR1) chromophore side group, is independent of the thickness and the percentage of the chromophore, and is almost constant at 6 x 10(-8) m(2)/s. The thermal diffusivity of the electrically poled NLO polymer thin film is independent of the intensity of the applied electric field.