Abstract: Objective To observe the toxicity in the lung by three dimensional conformal radiotherapy (3DCRT) combined with concurrent chemotherapy forⅢ stage non-small cell lung cancer(NSCLC). Methods From Sep. 2006 to Jul. 2008, 47 patients with NSCLC were treated with 3DCRT combined with concurrent chemotherapy of navelbine+cisplatin(NP).The patients received conventional fractionated radiation. The prescription dose is 60Gy. Univariate and multivariate analysis were performed to test the association between radiation pneumonitis(RP) and dosimetric and clinical factors. Results (1)Three patients showed complete remission(CR),forty-two patients partial remission(PR)with an overall response rate of 95.74% and the 1-year survival of75.78%.Of these 47patients, 45 developed radiation pneumonitis(RP): 20 grade 1, 17 grade 2 and 8 grade 3, there was no radiation pneumonitis of grade 4. (2)The correlative dosimetric factors included;mean lung dose(MLD),normal tissue complication probability(NTCP)and volume of lung which received≥5,15and 20Gy.Of the clinical factors,the volume of gross tumor volume(GTV)was associated with RP.Only mean lung dose was independently associated with RP according to the multivariate analysis. Conclusion Dosimetric factors(MLD,NTCP,lungV5,V15,V20)were pronounced predictors of radiation pneumonitis(RP) after 3DCRT combined with concurrent chemotherapy of navelbine+cisplatin(NP) for Ⅲ stage NSCLC .According to the multivariate analysis,mean lung dose(MLD) was independently associated with RP.