A 7-year-old female maltese dog with anorexia and vomiting for a week, and left axillary and perineal mass was admitted to Gyeongsang National University Veterinary Teaching Hospital. CBC revealed mild leukocytosis and severe anemia. Abdominal radiography revealed multiple gas opacities in upper abdomen. On ultrasonography, large amount of echogenic free abdominal fluid were detected. In addition, There are enhanced peritoneal strip sign (EPSS) indicating peumoperitoneum in the upper abdomen. Fine needle aspiration of the mass was performed and the result was mast cell tumor. Duodenal perforation caused by mast cell tumor was identified on laparotomy.