This paper describes the measurement of drug density which is proposed for a newly developed ultrasound (US) drug delivery system (DDS). In this DDS, drug is carried in microcapsule and delivered to the lesion of a body. Here, an experiment was performed to estimate a microcapsule density that corresponds to the drug density from the brightness on an US B-mode image. The results indicate that the brightness on a US B-mode image is an index for a microcapsule density and that the our proposed method has the possibility to measure a drug density non-invasively.