The aim is to investigate the feasibility of shrinking field technique after 40 Gy for stage III non-small cell lung cancer (NSCLC) during radiation therapy. Eighty-seven consecutive patients treated with intensity-modulated radiation therapy or three-dimensional conformal radiation therapy were enrolled in this study. (18)F-fluorodeoxyglucose positron emission tomography/computed tomography ((18)F-FDG PET/CT) scanning was performed prior to treatment and repeated after 40 Gy, and the delineation of target volume was based on fused images of PET and CT. After 40 Gy of conventional fractionated radiotherapy to the initial planning target volume (PTV), a boost of 19.6-39.2 Gy was delivered to the shrunken PTV through late course accelerated hyperfractionated radiotherapy, and the median total dose was 66.0 Gy (range, 59.6-79.2 Gy). Gross tumor volume (GTV) and PTV regressions were recorded, and prescription doses with or without shrinking field were calculated. Local recurrence patterns were investigated through follow-up. The tumor volumes regressed in 84 (96.6%) patients and increased in 3 (3.4%) patients after 40 Gy. The mean GTV and PTV reduction was 38% (range, -13-95%) and 30% (range, -5-95%). Mean total prescription dose escalated from 62.0 Gy to 68.5 Gy through shrinking field technique. The median follow-up was 17 months, ranging from 5 to 46 months, and the 1- and 2-year overall survival rates in our study were 74.7% and 34.6%. The response rate was 79.5%, and radiation toxicity was acceptable. Tumor progression occurred in 67.8% (59/87) patients. Numbers of patients who had outfield, infield and both infield and outfield recurrences were 3 (3.4%), 26 (29.5%), and 3 (3.4%), respectively. In conclusion, significant tumor regression was observed after 40 Gy, and radiation dose escalated after shrinking field with acceptable toxicity and outfield relapse. Shrinking field radiotherapy based on (18)F-FDG PET/CT after 40 Gy was safe and feasible for stage III NSCLC.