In this paper, the process-affected zone in ultrasonically embossed thermoplastic substrates is investigated both numerically and experimentally. Commercialization of microfluidic devices challenges the need for high-speed manufacturing of plastic chips. Ultrasonic embossing is considered as an alternative method since the cycle time can be as low as a few seconds per chip while keeping the cost relatively low. To examine the ultrasonic embossing process, experiments were carried out to replicate 200 mu m wide and 150 mu m high straight channels on 3 mm thick polymethylmethacrylate (PMMA) substrates. The mold was fabricated by milling on aluminum. The features could be embossed by applying an 85 N static force at 28 kHz ultrasonic vibration and 10 mu m amplitude for 5 s at room temperature with replication rates of 99.5 % and 100 % for the width and the depth, respectively. During the experiments, a clearly visible process-affected zone typically bounded by a half-circle with the center at the channel axis was observed. It was proven that the process-affected zone was bounded by the isothermal surface at the glass transition temperature of the substrate material (107 degrees C), both numerically and experimentally. It was also shown that the composition of the substrate material remains unaffected within the process-affected zone.