We have developed a non-invasive and quantitative method to estimate the size of RBC aggregates by analyzing ultrasonic backscattering properties from blood vessel lumen and comparing those with the theoretical scattering characteristics. In the present study, we aimed to improve accuracy by calculating and correcting the attenuation in the blood vessel lumen. The attenuation in the blood vessel lumen was calculated with the spectra acquired from two different depths. In the basic experiments using microparticles, the estimated size tended to approach the true size irrespective of the concentration removing the propagation attenuation component by the proposed method.