A 3-month-old female with ventricular septal defect underwent definitive repair. In the operation, a venting tube was inserted through the incised patent foramen ovale. The ventricular septal defect was closed using a patch and the incised patent foramen ovale was directly closed. A transesophageal echography showed no abnormal findings in the operating room. On 1st postoperative day, the patient's heart rate and pressure of the superior vena cava increased while blood pressure decreased. A transthoracic echocardiography revealed an atrial septal hematoma. After conservative therapy, the heart rate and pressure of the superior vena cava gradually decreased while blood pressure increased. The atrial septal hematoma disappeared spontaneously. The patient was discharged on 16th postoperative day. The atrial septal hematoma appears to have a good prognosis, but sometimes can be poor resulting and death. Meticulous and constant observation is required to detect an atrial septal hematoma.