Abstract: In this paper a new method of in vivo estimation of ultrasonic attenuation-Amplitude Difference Method is described. Amplitude Difference Method (AD method) can eliminate the effect of system deviation and lessen the effect of beam radiation diffraction. It can be applied in conjunction with many kinds of B-scanners. To verify the AD method, this paper introduces a new method to make tissue phantom and a special apparatus. The results show that the AD method is useful in the diagnosis of diffuse liver diseases, tumors and cancers and in the appraisal of the health and the stages of fetus.