In this paper, we introduce new diagnosis tool to observe carotid artery based on ultrasonic volume data. The main components and applied algorithms of the developed diagnosis tool are explained. As one of main components, the semi-automatic segmentation method includes an effective speckle reducing filter and an automatic ROI tracking scheme. Furthermore, we present the reconstruction method that is effective for Y-typed carotid artery and the navigation path generation method that applies interpolation of medial points of ROI. To support the objective diagnosis, we provide the automatic measurement method of artery's diameter, To show usefulness of the developed tool, we constructed 3D model for carotid artery of 34-year-old person and the diameter of carotid artery was automatically measured.