Abstract: The image characteristics, such as enhancements, shadows and lateral shadows, posterior to a tumor in a B-Mode diagnostic ultrasongraphy, are studied. The tumor is simplified as a homogeneous cylinder in an inhomogeneous medium. With a ray method, we derive an intensity formula for the region behind the cylinder. Then, considering the influence of sound attenuation and that of the time gain compensation of the B-Mode displayer, we simulate five images with a computer, which correspond to our experiments conducted with a tissue-equivalent phantom. The theoretical images seem to be in good qualitative consistency with the experimental results.