Abstract: Objective To observe the incidence of acute radiation pneumonitis in non-small cell lung cancer(NSCLC) patients treated with three-dimentional conformal radiotherapy (3DCRT), and to investigate the correlation between dosimetric parameters and radiation pneumonitis. Methods From June 2010 to December 2010, 68 NSCLC patients treated with 3DCRT were retrospectively reviewed. The following dosimetric parameters were calculated from dose-volume histogram (DVH) system: total dose, mean lung dose(MLD), V5, V10, V15, V20, V25, V30, V35, V40, V45 and V50. Univariate and multivariate analyses were performed to determine the correlation between those factors and radiation pneumonitis. Results V5 was the independent factor significantly associated with radiation-induced lung damage(χ2=5.15, P=0.023). The incidence of radiation pneumonitis would be increased if V5 was more than 57%. Conclusion Except common parameters, such as V20, V30, MLD, etc., V5 should also be considered in assessing radical 3DCRT plan.