Coronary artery injury after blunt chest trauma is very rare, but this can result in a serious acute myocardial infarction. Coronary artery dissection is an uncommon complication of thoracic injuries. We report a case of a 17-year-old male who was presented with an anterior myocardial infarction following blunt chest trauma after a bicycle accident. His coronary angiography revealed aneurysmal dilatation with dissection of the distal left main stem coronary artery. Intravascular ultrasound showed a dissecting flap at the left main stem coronary artery. The patient was treated conservatively and discharged without serious sequelae. When symptoms and electrocardiographic findings are compatible with acute myocardial infarction, careful evaluation is important in patients with thoracic injuries for proper management. If the patient is stable, medical therapy may be appropriate. But early intervention should be considered in the presence of ongoing myocardial ischemia.