Pressure measurements are used in many industrial applications. Typically, the pressure sensing device must be in direct contact with the pressurized media. However, there are several problems associated with placing these devices inside pressure vessels. External clamp-on ultrasonic transducers are an attractive alternative. In this research, a pressure measurement based on ultrasonic waves reflected from the inside surface of the vessel wall has been investigated. Results showed that the signal amplitude could be related to vessel pressure. Because signal amplitude is sensitive to other factors, a potential strategy for integrated calibration has also been considered.