Abstract: Objective To retrospectively analyze physical parameters and clinical factors for radiation pneumonitis(RP) in stage Ⅲ non-small cell lung cancer(NSCLC) patients treated with three-dimensional conformal radiotherapy(3D-CRT). Methods From January 2001 to December 2008, 203 NSCLC patients were treated with radical 3D-CRT, 163 males and 40 females; Median ages was 63-year-old(30~83); 21 patients were suffered from chronic obstructive pulmonary disease(COPD) medical history; 79 cases were in ⅢA stage and 124 cases were in ⅢB stage; 70 cases were treated with single radiotherapy and 133 cases with chemoradiotherapy, and median equivalent dose was 6 200 cGy (5 000-7 800 cGy). RP was evaluated with RTOG standard, and physical parameters of DVH was used to estimated RP grade≥2 and RP grade≥ 3. SPSS 13.0 software was used for statistic analysis. Results After 3DCRT, the rates of RP grade≥2 and grade≥3 were 32%(65/203) and 20.7%(42/203). Spearman correlation and single factor Logistic analysis showed that the mean lung dose, V5-V40 of total lung, COPD history, fractional radiation style, radiotherapy dose, GTV and GTV/volume of total lung were correlated with RP grade≥2 and grade≥3 (P<0.05); Logistic multivariate analysis revealed that V25 was the independent risk factor for RP grade ≥2 and grade ≥3. The optimal cutoff value for lung V25 was 29% in the receiver-operating characteristic (ROC) curve. COPD was the independent risk factor for RP grade ≥2. Conclusion DVH parameters were associated with the occurrence of RP. V25 might be the most effectively independent risk factor for RP grade≥2 and grade ≥3. COPD was the independent risk factor for RP grade≥2 in patients with non-small cell lung cancer.