To elucidate the dependence of diffusion coefficient of disperse dye in poly(ethyleneterephthalate) (PET) on concentration, p-nitroaniline(PNA) was used as a model penetrant of one dye over the wide range of concentration at 70, 75, 80, 85, 90 and 95$^{circ}C$. Diffusion coefficients were measured by cylindrical film roll method. The depence of diffusion coefficient on penetrant concentration was clearly observed (Fig. 2), and empirically represented by the equation (I) for the lower concentration (up to about 8${ imes}$10-2mol/kg) and (2) for the higher concentration. where C is concentration of penetrant and n and$delta$ are constants. PNA penetrated into an easily diffusible amorphous region at the lower concentration. At the higher concentration of PNA, the penetrant diffused into more closely packed amorphous part without changing the total amounts of amorphous region available for dyeing. It was suggested that the increase of diffusion coefficient was caused by the minute qualitative change of amorphous region of PET, which could not be detected by the IR spectrum (Table 2), small angle X-ray scattering and dichroism of the dyed film. With increasing PNA concentration the decrease of relaxation modulus of the PET film was observed (Fig. 6), and this tendency corresponded to the increase of diffusion coefficient mentioned above (Fig. 2).