The concept of a wireless sensor for injection molding machine cavity pressure measurement is proposed. The pressure information inside the cavity can be sensed by means of a series of ultrasonic pulses. The current pressure is obtained simply multiplying the number of pulses with a preset threshold value. Instead of using any external power supply, the sensor is self-powered by a piezoelectric stack that extracts energy from the melt pressure changes and converts it into electrical energy. Theoretical background of ultrasonic pulses and micromechanical switches are introduced, in view of their applications in the proposed design. Preliminary experiments have shown that pressure change of 20 MPa within 1 μs could be detected wirelessly by a single ultrasonic pulse. Further work will be done to validate the proposed design.