Specimens from amorphous copolyester (aCOP) sheets of different composition and molecular mass characteristics were subjected to instrumented tensile impact loading in the speed range of 2 - 3.7 m/s at ambient temperature. Stress oscillation was observed in the necking phase, especially for the aCOP with smaller entanglement density. It was found that the appearance of stress oscillation is limited to a given frequency or strain rate range (estimated by the ratio of the incident speed and clamped specimen length). Necking occurred by cold drawing which was evidenced by the shape recovery of the broken specimens when heated above the glass transition temperature (Tg approximate to 90 degrees C). According to polarized light microscopy and differential scanning calorimetry (DSC), the stress oscillation was generated by diffuse shear banding without crystallization. This is the first report according to the authors＇ knowledge on the stress oscillation phenomenon in amorphous polymers in which crystallization was not involved.