PET/nylon 6 blends were prepared in the presence of p-toluenesulfonic acid (TsOH) and analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and X-ray diffraction (XD). As ester-amide interchange reaction proceeds, Tm of each component in the blends was decreased and crystallization temperature on cooling was increased. Especially, Tm of nylon 6 component was decreased more rapidly than that of PET component with the increase of blending time. This finding is explained by the reduction of hydrogen bonding interaction of nylon 6 resulting from interchange reaction. SEM observation and XD analysis showed that miscibility was increased with the progress of inter-chain reaction.