To establish an evaluation index for vascular endothelial function, we developed an ultrasonic probe that can measure changes in blood pressure and blood vessel diameter at the same position in the radial artery. Based on phantom experiments, the pressure waveforms measured using the piezoelectric effect of the ultrasonic probe element and those using a pressure sensor exhibited a high correlation (correlation coefficient: r = 0.979 - 0.999). We confirmed the continuous measurement of the relationship between changes in blood pressure and the diameter from the in vivo experiments. We could then estimate the changes in viscoelasticity by calibrating the output from the probe element to the absolute blood pressure values in advance. The average coefficients of variations were 0.02 and 0.24 for elasticity and viscosity, respectively. This study demonstrated the possibility of measuring changes in the viscoelastic moduli of the radial arterial wall due to flow-mediated dilation using the developed ultrasonic probe.