Mesenchymal hamartoma is an uncommon benign tumor usually found in childhood, especially during the first two years of life. The tumor consists of a solid component and multiple cyst. Without treatment, these lesions can grow to an enormous size. We experienced a case of mesenchymal hamartoma which was found incidentally, in a 8 month old girl who had ascending cholangitis after Kasai operation due to biliary atresia. On abdominal ultrasonography and CT, there was a well-defined homogeneous small echogenic mass in the right lobe of the liver with cirrhosis. Five months later, she underwent liver transplantation. Gross picture of the resected liver showed a dark greenish pigmented solid mass in the right lobe of the cirrhotic liver. Microscopic findings showed reactive mesenchyma and epithelial overgrowth.