Abstract: A pattern called "speckle" appears in the B-mode images. The purpose of this paper is to make clear the characteristics of the ultrasonic speckle phenomena. The relationship between the outbreak of the speckle and the scatterers' density is investigated. We have quantitively examined speckle region, where the speckle is in domination and led to the extimated density d and correlation coefficient α. Theoretical and experimental results show that the speckle resion arises when more than about 18 scatterers exist in the volume of the resolution limiting of the system.