Abstract: In this paper, 17 cases of malignant mesothelioma and 20 cases of metastatic carcinoma of the pleura and peritoneum were pathologically compared by the use of electron microscopes. It was found that malignant mesothelioma is characterized by the presence of long bush-like microvilli on the surfaces of tumor cells and intracellular lumina and abundant tonofilaments in the cytoplasm of tumor cells. In contrast, the metastatic carcinoma studied shows relatively short, simble microvilli and scanty or no tonofilaments in metastatic adenocarcinoma. Metastatic adenocarcinoma showed microvillous rootlets （17/20）, lamellar bodies （6/20）, cilia（12/20）, glycocalyceal bodies （17/20） and mucinous secretory granules（18/20）, which were absent in malignant mesothelioma. For the purpose to provide an accurate diagnosis for the clinician, it is necessary to study electron-microscopically for differentiating malignant mesothelioma from difficult metastatic carcinoma of pleura and peritoreum. In addition, ultrastructural diagnostic criteria of the malignant mesothelioma have been investigated too.