The ultrathin sections manufactured by improved fixative for biological samples of electron microscope and new embedding media were used to study the ultrastructure changes of the normal and MBV-infected epithelial cells of the hepatopancreas under JEM-1200. The results indicated there was one or more intranuclear inclusion bodies and a great number of virions in the karyoplasm of the infected cells. As the number of inclusion bodies and virions increased，the unclear chromatins agglutinated and the nucleolar migrated，the nuclear membrane partially dilated，disrupted or even completely disappeared. At last，the whole nucleus would disintegrate and drift the inclusion bodies out of the nucleus. In this periods，the oedema and necrosis of cell plasms，the myeloid deterioration of endoplasmic reticulum，the hyperplasia of agranular endoplasmic reticulum and lipiddroplets，the down-wass-like deterioration of mitochondrias and the secondary bacterial mixed infections also be found in the studies. The authors consider that the large number of intranuclear inclusion bodies and MBV (existed in the karyopalsm)，which caused the severe cytopathologic changes，could serve as one of the characteristic indication in diagnosing the viral disease of P. monodons. The bacterial mixed infection is one of the important auxiliary diagnostic indication. The lipoidosis and pathologic changes of mitochondrias in epithelial cells of the hepatopancrea may have effects on the normal growth of the surviving P. monodons.