The retrospective analysis was performed on 37 patients with stage III non small cell lung cancer who received the radiotherapy from Feb. 1986 to Dec. 1990 at the Dept. of Radiation Oncology, National Medical Center. This analysis, with 29 patients (78.4%) having been followed from 10 to 60 months, was done to know the survival rate and significant prognostic factor. The actuarial 2, 5-year survival rates were 20.6%, 6.9% in our all patients and Median survival time was 10 months. Of patients with KPS(Karnofsky performance status) greater than 80%, the 2, 5 year survival rate and median survival time were 29.2%, 9.7% and 13 months, respectively. The 2-year survival rate and median survival time of patients with KPS less than 80% were 13.7% and 7 months, respectively. The survival difference according to performance status was statistically significant(29.2% vs. 13.7%)(p<0.05). In stage IIIa, the 2, 5-year survival rate and median survival rate and median survival time were 29.2%, 9.7% and 12 months, respectively. The 2-year survival rate and metian survival time of stage IIIb were 8.6% and 10 months, respectively. The survival difference between stage IIIa and IIIb did not show statistical significance(p>0.1). Of the prognostic factors, the difference of survival rate by initial performance status was statistically significant (p<0.05). But the difference of survival rates by pathologic cell type, stage, total radiation dose, radiotherapy response, and combination with chemotherapy were not statistically significant.