We have proposed an evaluation method of the viscoelastic parameters of blood walls by measuring a relationship between radial arterial pressure and diameter to evaluate vascular endothelial functions. In our previous study, those were measured at different position, which caused timing errors because of the difference of pulse wave velocity during a heartbeat. In the present study, a novel probe was developed to measure both radial arterial pressure and diameter at identical position. One cord connecting the central piezoelectric element of the linear array probe and the ultrasonic diagnosis equipment was cut off to measure the arterial pressure. The output from the element was amplified 100 times, filtered with a low pass filter, and acquired with an oscilloscope. The output was obtained as the differential form of the blood pressure waveform. To obtain the blood pressure waveform, the output was integrated and calibrated with the systolic and diastolic blood pressures measured by the tonometry. The arterial diameter was measured using the other 127 elements with the ultrasonic diagnosis apparatus by the phased-tracking method. The hysteresis loop between the diameter and pressure during a heartbeat, which reflect the viscoelasticity of the blood wall, was successfully confirmed.