AbstractA series ofN‐alkyl‐p‐toluenesulfonamides (ATSA) were synthesized and identified. The measurement of solution viscosity of poly(ethylene terephthalate) (PET) proved that ATSA does not react with PET during the melting process of PET/ATSA mixtures, and consequently does not cause any decrease of molecular weight of PET. The compatibility of ATSA with PET was investigated by polarized light and scanning electron microscopy. Phase separation could not be found to a 0.5 μm level. The investigation of crystallization of PET/ATSA mixtures by means of differential scanning calorimetry revealed that ATSA significantly accelerates the crystallization rate of PET, and that ATSA is more effective when increasing the length of alkyl chains within ATSA. The increase of crystallization rate of PET is proposed to be caused by the improvement of molecular motion of chains, which can be proved by the depression of glass transition temperature when ATSA is