A patient who had ovarian mass, ascites and elevated CA 125 levels was treated with presumptive diagnosis of ovarian cancer. After pathological examination, she was finally diagnosed as malign peritoneal mesothelioma. Although exposure to asbestos was considered, the diagnosis was not revealed before surgery. Despite similarity of clinical features, ovarian cancer and malign peritoneal mesothelioma have different treatment options and long-term outcomes. The aim of this case report is to describe the process of differential diagnosis between malign peritoneal mesothelioma and ovarian cancer.