The endothelial dysfunction is considered to be an initial step of atherosclerosis. Additionally, it was reported that the smooth muscle, which constructs the media of the artery, changes its characteristics due to atherosclerosis. Therefore, it is essential to develop a method for assessing regional endothelial function and mechanical property of arterial wall. There is a technique to measure the transient change in diameter of the brachial artery caused by flow-mediated dilation (FMD) after release of avascularization. For more sensitive and regional evaluation, we developed a method to measure the change in elasticity of the radial artery due to FMD. In this study, the transient change in the mechanical property of the arterial wall was further revealed by measuring the stress-strain relationship during each heartbeat. The minute change in thickness (strain) of the radial arterial wall during a cardiac cycle was measured using the phased tracking method. At the same time, the waveform of blood pressure at the radial artery was continuously measured with a sphygmometer. Transient change due to FMD in the stressstrain relationship during a cardiac cycle was obtained from the measured strain and blood pressure to show instantaneous viscoelasticity. From the results, the stress-strain relationship shows the hysteresis loop. The slope of the loop decreased due to FMD, which shows that the elastic modulus became lower, and the increasing area of the loop depends on the ratio of the loss modulus (depends on viscosity) to the elastic modulus when the Voigt model is assumed. These results show a potential of the proposed method for thorough analysis of the transient change of viscoelasticity due to FMD.