Thyroid blood flow related with thyroid function is of great importance in diagnosing thyroid gland disease. However, there has no report on such observations of thyroid blood flow dynamics, especially non-invasively and quantitatively. In this study new developed ultrasound duplex scanning system was used to measure the superior thyroid blood flow and analyze the hemodynamics in 105 cases of solitary thyroid tumors, including 30 cases of thyroid cancers. Superior thyroid blood flows on afferent portion obviously increased both in benign and malignant tumor cases, but clear difference between them could not be found. On the other hand, in concern with the blood flows on inafferent portions, malignant thyroid tumor cases showed remarkable increase with statistical difference. The blood flow ratio between afferent and inafferent portions was considered to distinguish between benign and malignant thyroid tumors as quantitative criteria. The assumption that the tumors whose blood flow rate was below 3.0 is malignant led the fine result that accuracy was 79.8%, sensitivity; 71.5%, specificity; 81.5%. This quantitative criterion requires no surgical technique, therefore, this is useful for the tissue characterization of thyroid tumors.