Although brachial plexus injury occurring during multitrauma is frequent in adults, it is rarely observed in childhood. The most common cause of pediatric traumatic brachial palsy is motor vehicle accidents followed by pedestrian struck. Generally, phrenic nerve palsy accompanying brachial plexus trauma is observed in 10% to 20% of cases, but it is overlooked because unilateral injuries are frequently asymptomatic. Severe unilateral phrenic nerve palsy accompanying brachial plexus avulsion is very rare. Here, we present a pediatric case of unilateral phrenic nerve palsy associated with respiratory distress and brachial plexus avulsion due to multitrauma.