Hemothorax is a rare disease in children but is a result of thoracic trauma, iatrogenic causes, cardiorespiratory diseases, malignancy, sequestration and so on. A previously healthy 7-yr-old boy patient experienced left chest wall pain that aggravated by deep breathing, with fever and cough, and came to the emergency room. There were no specific findings, so he returned home. Ten days later, he hospitalized with enteritis, and experienced left chest wall pain again. But symptom was improved, so he was discharged. One week later, because of abruptly aggravated chest pain and suspected of pleural effusion, he came to the emergency room. Chest CT showed atelectasis and consolidation in left lung with internal low attenuation in left hemothorax, and large amount of multiloculated left pleural effusion. The chest tube was inserted and the hemothorax was confirmed. After chest tube drainage, follow-up chest CT showed lentiform high attenuated nonenhancing lesion in left hemithorax posterior portion. The probable diagnosis based on the imaging findings was extralobar pulmonary sequestration with hemorrhagic infarction. After urokinase treatment, pleural effusion was improved and he was discharged. After 10months, follow-up chest CT showed no hemithorax and normal anatomy. Therefore, patients with nonspecific chest pain should be suspicious of pulmonary hemithorax.