Background: Despite a dramatic increase in utilization of stereotactic body radiation therapy (SBRT) for non-operative treatment of clinical stage I NSCLC over the last decade, there are no reported clinical trials or large scale comparisons with standard conventionally fractionated radiation (CFRT). We sought to compare overall survival (OS) between these modalities using the National Cancer Data Base (NCDB). Methods: Patients with clinical stage I (AJCC 6^th edition T1-T2N0M0) NSCLC who underwent definitive non-operative treatment with either SBRT or CFRT from 1998 to 2010 were identified from the NCDB. Cox regression was used to assess the impact of patient and treatment variables on OS. Inverse probability adjusted Kaplan-Meier survival curves were calculated to assess differences in OS while adjusting for baseline imbalances between the treatment groups. Results: A total of 19373 patients met inclusion criteria (5944 SBRT, 13429 CFRT). Median follow-up was 18.7 months. On multivariable analysis, improved OS was independently correlated with younger age, female gender, non-Caucasian race, lower Charlson comorbidity score, smaller tumor size, use of chemotherapy, treatment at an academic facility, and was most strongly associated with use of SBRT (HR .775, 95% CI, .737 to .817). Chemotherapy was delivered in 32.9% of CFRT patients and 4.2% of SBRT patients. Chemotherapy was correlated with improved OS on multivariable analysis for CFRT but not SBRT. In an adjusted Kaplan-Meier comparison, SBRT was associated with a significant increase in median (29.8 v 26.2 months) and 3-year (42.3% v 36.4%) OS compared with CFRT (P < .0001). Conclusions: In a large population-based analysis of outcomes in a modern cohort of patients with clinical stage I NSCLC treated with definitive non-operative therapy, SBRT was associated with a significant improvement in OS compared with CFRT.