Background: Body mass index (BMI) has a U-shaped association with lung cancer risk. However, the effect of BMI on prognosis is controversial. This retrospective study aimed to investigate the effect of BMI on the survival of patients with stage I non-small cell lung cancer (NSCLC) after surgical resection. Methods: In total, 624 consecutive stage I NSCLC patients who underwent radical resection were classified into four groups according to their BMI: underweight (BMI < 18.5 kg/m(2)), normal weight (BMI = 18.5-22.4 kg/m(2)), overweight (BMI = 22.5-28.0 kg/m(2)), and obese (BMI > 28.0 kg/m(2)). The effect of BMI on progression-free survival (PFS) and overall survival (OS) was estimated using the Kaplan-Meier method and Cox proportional hazards model. Postoperative complications in each group were analyzed using the Chi square test or Fisher's exact test. Results: A univariate analysis showed that PFS and OS were longer in the overweight group than in other groups (both P < 0.05). A multivariate analysis showed that OS was longer in the overweight group than in other groups (compared with the other three groups in combination: hazard ratio [HR] = 1.87, 95% confidence interval [CI] 1.30-2.68, P = 0.003; compared with the underweight group: HR = 2.24, 95% CI 1.18-4.25, P = 0.013; compared with the normal weight group: HR = 1.58, 95% CI 1.07-2.33, P = 0.022; compared with the obese group: HR = 2.87, 95% CI 1.48-5.59, P = 0.002), but PFS was similar among the groups (HR = 1.28, 95% CI 0.97-1.68, P = 0.080). A subgroup analysis showed an association between being overweight and prolonged OS in patients at stage T1a (P = 0.024), T1b (P = 0.051), and T2a (P = 0.02), as well as in patients with a non-smoking history (P = 0.001). Overweight patients had lower rates of postoperative complications, such as respiratory failure (compared with the underweight and obese groups: P = 0.014), myocardial infarction (compared with the obese group: P = 0.033), and perioperative death (compared with the other three groups: P = 0.016). Conclusions: Preoperative BMI is an independent prognostic factor for stage I NSCLC patients after resection, with overweight patients having a favorable prognosis.