Anesthesiologists may encounter pulmonary thromboembolism during the perioperative period. However, the diagnosis of pulmonary thromboembolism is difficult because clinical symptoms and signs are not specific, and specialized diagnostic tools are not readily available in the operating room. We report a case of pulmonary thromboembolism during modified radical mastectomy. A 52 year old female patient underwent modified radical mastectomy under general anesthesia. 90 minutes after induction there were a sudden increase of peak airway pressure and a decrease of oxygen saturation. We treated symptomatically under the impression of pulmonary edema during operation. After operation, pulmonary thromboembolism was diagnosed and treated successfully with oxygen therapy and anticoagulant therapy. After 26th days of operation she was no longer suffered from hypoxia. At the 36th day after operation, she was discharged without any sequelae.