Elastic modulus to the circumferential direction of carotid artery is important to evaluate the degree of arteriosclerosis. Previous methods have utilized the magnitude of systaltic movement of carotid wall to estimates the elastic modulus. Because the magnitude depends on the elastic modulus of surrounding tissues around carotid artery as well as that of carotid artery itself, the previous methods results in overestimates of carotid elastic modulus. We then modeled both carotid artery and surrounding tissues with a two-layered combinational cylinder from the view point of Theory about Strength of Materials. Utilizing the systaltic movement of a point in surrounding tissues as well as that of carotid wall, we estimate the "self-"elastic modulus of carotid artery by excluding influence of surrounding tissues. We further measured carotid elastic modulus in vitro for animal by tensile test. The results agree with our in vivo estimation.