Background: Postoperative radiation therapy (PORT) remains the critical therapy for stage III non-small cell lung cancer (NSCLC). Radiation induced lung injury (RILI) is common and affects the clinical outcome. Proton therapy (PT) is a new-style radiotherapy with accurate distribution of curative dose to tumor and increased organ-at-risk (OAR) sparing, which potentially decrease the incidence of RILI. Intensity modulated proton therapy (IMPT) is more flexible and conformal one.Case Description: In the case, we report a 47-year-old man with stage III locally advanced lung adenocarcinoma developing RILI after IMPT. The man had no chronic pulmonary disease before. After 6 cycles every three-week of postoperative adjuvant chemotherapy (pemetrexed, carboplatin), he sequentially received 50 GyE of IMPT in 25 fractions. About 7 weeks after IMPT, grade 2 RILI was developed with the manifestation of focal pulmonary consolidation and ground-glass attenuation. Steroid therapy was delivered and the pneumonias absorbed slightly with chronic scarring and fibrosis left over.Conclusions: RILI after IMPT is not commonplace especially under the circumstance where the patient had no chronic lung disease and the proton dose was conservative. The patient manifested as the early developed acute exudation and fibrosis stage. Moreover, the injury was so refractory that fibrosis was developing in spite of active steroid therapy. Based on the case, we suggested that more exploration of proton induced and evaluation before IMPT are deserved.