Pseudocysts of lung by blunt chest trauma are rare lesion with 1 to 2 % incidence. The symptoms are nonspecific and misled or confused with lung abscess or congenital cystic disease of lung occasionally. Diagnosis is not difficult by radiographic findings and history of blunt chest trauma. There courses are benign and spontaneously absorbed and require no specific treatment mostly. Recently, we had the one case of 8 year-old female with traumatic lung cyst and its cavities filled the entire right lower lobe. The destruction of lung was severe, so, right lower lobectomy was done and recovered, discharged without event.