Real-time process monitoring of the fabrication process of microfluidic devices using a polymer injection molding machine was carried out using miniature ultrasonic probes. A thick piezoelectric lead-zirconate-titanate film as an ultrasonic transducer (UT) was fabricated onto one end of a 4-mm diameter and 12-mm long steel buffer rods using a sol gel spray technique. The center frequency and 6 dB bandwidth of this UT were 17 MHz and 14 MHz, respectively. A signal-to-noise ratio of more than 30 dB for ultrasonic signals reflected at the probing end was achieved. The probe can operate continuously at 200 degrees C without ultrasonic couplant and cooling. Clear ultrasonic signals were obtained during injection molding of a 1-mm-thick part having test patterns on its surface. Shrinkage of the molded part and part detachment from the mold were successfully monitored. Surface imperfections of the molded parts due to a lack of the sufficient holding pressure is discussed with regard to the ultrasonic velocity obtained. The presented ultrasonic probes and technique enable on-line quality control of the molded part by optimizing the holding pressure and improvement of process efficiency by reducing the cycle time. (c) 2005 Society of Plastics Engineers.