We developed a single ultrasound probe to simultaneously measure blood pressure and changes in the diameter of the radial artery to estimate the wall viscoelasticity during flow-mediated dilatation (FMD). This probe can be used for the early diagnosis of arteriosclerosis. This paper introduces the pulse transit time method to accurately measure changes in blood pressure during FMD. Using the single ultrasound probe and the proposed method, in vivo experiments involving three subjects were conducted, and reasonable results on blood pressure were obtained. Thus, the usefulness of the pulse transit time method was experimentally confirmed.