Objective To investigate the irfluence of PET/CT positioning on the effect of late-course accelerated hyperfractionated and three-dimensional conformal radiotherapy for stage Ⅲ lung squamous cell carcinoma.Methods 79 stage Ⅲ lung squarnous cell carcinoma patients were enrolled and divided into PET/CT group(n =37 cases,PET/CT positioning) and general CT group (n =42 cases,general CT positioning) according to the way of positioning.The conventional fractionated 3 DCRT( about 40 Gy) was used in previous 2/3 time and hyperfractionated( about 1.5 Gy) was used in later 1/3 time.Both groups used chemotherapy as adjuvant therapy.Full course of the treatment was a month or so.And the clinical efficacy and a variety of adverse reactions of the two groups was observed.Results The total rate of remission in PET/CT group were 73.0％,and 71.4％ in general CT group,and there was no significant difference in the two groups (x2 =1.347,P ＞ 0.05 ).The major radiation injury were radioactive pulmonary injury and tracheal damage,incidence rates were 56.8％and 57.1％ in acute stage,and there was no significant difference in the two groups( x2 =2.178,P ＞ 0.05 ) ; but in late stage 69.0％ in general CT group was significantly higher than 62.2％ in PET/CT group(x2 =4.142,P ＜0.05).Ater followed-up 1 year,the recurrence rate of hillar and mediastinal lymph node in PET/CT group was 43.7％,lower than that of the general CT group( 59.4％ ) ( x2 =4.732,P ＜ 0.05 ).Conclusion PET/CT positioning in late-course accelerated hyperfractionated and three-dimensional conformal radiotherapy for stage Ⅲ lung squamous cell carcinoma could reduce late stage radioactive pulmonary injury and tracheal damage,lower the recurrence rate of hillar and mediastinal lymph node.