Abstract: Objective At present, the dosimetric parameters used to predict radiation pneumonia（RP） are almost based on three-dimensional conformal radiotherapy（3D-CRT）. Intensity modulated radiation therapy（IMRT） is more recommended for radical radiotherapy of lung cancer, but the dosimetric parameters for RP prediction based on IMRT have not been established. This study aims to find the best predictor of RP in non-small cell lung cancer（NSCLC） patients receiving radical IMRT. Methods NSCLC patients who received radical IMRT in the Department of Radiotherapy of Guangdong Provincial People's Hospital from 2014 to 2019 were retrospectively analyzed. The primary endpoint was RP of degrees 2 or more（RP2）. The lung dosimetric parameters were V5, V10, V20, V30, V40 and mean lung dose（MLD）. The volume of normal lung was defined as bilateral lung minus gross tumor volume （Lung-GTV）, bilateral lung minus clinical target volume （Lung-CTV）, bilateral lung minus planning target volume （Lung-PTV）. The correlation between RP2 and lung dosimetric parameters was analyzed. Results A total of 73 patients were included in the analysis. The incidence of RP2 was 14.9%. Age > 60 years old and sequential chemoradiotherapy were the high risk factors of RP2. Regardless of the lung definition, V10 was significantly associated with RP2. The correlation between V10 Lung-GTV and RP2 was the strongest（P=0.01, AUC=0.776）. Conclusions V10 was significantly associated with the development of RP2. V10 Lung-GTV was the best predictor of RP2 in NSCLC patients receiving IMRT.