Objective To analyse the long-term result and prognosis of 18 FDG PET/CT positioning three - dimensional conformal radiotherapy ( 3 DCRT ) for stage Ⅲ non - small cell lung cancer. Methods Sixty-four cases with stage Ⅲ non-small cell lung cancer (clinical stage Ⅲa- Ⅲb ) were randomly divided into two groups: PET/CT positioning three-dimensional conformal radiotherapy group (PET/CT group) and the conventional CT positioning three-dimensional conformal radiotherapy group (conventional CT group). In the PET/CT group, the target volume and critical organs were sketched according to PET/CT after fusion of the PET and the CT images; the treatment plan was worked out, then conventional fractionated 3DCRT ( total dosage around 40 Gy) followed by field-shrinked radiotherapy to a total dose of 65 Gy or sowas performed ;in the conventional CT group, the target volume and critical organs were sketched according to CT and 3DCRT were performed to the same total dose; All cases were treated with the TP scheme (paclitaxel 175 mg/m2,d1 ,cisplatin 40 mg,d2-4) adjuvant chemotherapy for 6 cycles after the radiotherapy. Results The followup rate was 100%. The number of patients who completed the 1-,2-and 5-year follow-up were 40,20 and 11 respectively ;The number of patients of the PET/CT group and conventional CT group were 23 and 17,11 and 9,7 and 4 respectively. Target volumes of 13 cases in the PET/CT group were changed. The complete remission and partial remission rates of the two groups were 13% 、66% and 19% 、53% (x2 = 0. 33, P =0. 564), respectively. The 1-,2-and 3-year local control rates of the PET/CT group and conventional CT group were84 % 、66% 、53 % an d72% 、59% 、44% ( x2 = 2.36, P = 0. 124 ) respectively. The1 -, 2-and 3-year survival rates were 72% 、34% 、22% and 53% 、28% 、13% (x2 =2. 46,P =0. 117) respectively. The level-1 and level-2 lungs' and trachea's late radiation injury of the PET/CT group and the conventional CT group were 28% and 53% ( x2 = 4. 14, P = 0. 042 ), respectively. The hilar and mediastinal lymph node recurrence rates of the PET/CT group were lower than those of the conventional CT group, were 3% ,25%(P = 0. 026) and 6%, 28% ( P = 0. 042 ), respectively. The main reason for treatment failure was distant metastasis both in the PET/CT group and conventional CT group,56% and 47% (x2 = 0. 56,P = 0. 453 ),respectively. Conclusions PET/CT, as a method of sketching the target of stage Ⅲ non-small cell lung cancer, can improve the radiation treatment plan, reduce the recurrence rate of hilar and mediastinal lymph nodes, meanwhile it can not improve the long-term survival rate; Distant metastasis was the main reason of failure.