To characterize tissues in atherosclerotic plaques, we have developed a method, the phased tracking method, for measuring the strain (change in wall thickness) and elasticity of the arterial wall. However, some tissues, such as lipids and blood clots, cannot be discriminated based only on elasticity due to the small difference in their elasticities. For more precise tissue characterization, we have measured the regional viscoelasticity. To assess the viscoelasticity, we proposed a method for measuring elastic moduli at multiple frequencies using remote cyclic actuation. In this paper, the viscoelasticity of an extracted human femoral artery was estimated in vitro from elastic moduli measured at multiple frequencies with remote actuation. Furthermore, the viscosity of human carotid arteries were measured in vivo by applying remote actuation at the brachial artery.