Objective: To assess the association between baseline body mass index (BMI) and risk of lung cancer in non-smoking males. Methods: A dynamic non-smoking male cohort was established on May, 2006. Baseline information on demography, lifestyle, such as smoking status and alcohol consumption, anthropometry, such as body height and weight, were collected during the baseline interview, and the information of newly-diagnosed lung cancer cases were also collected during the follow-up period. Multivariable Cox proportional-hazards regression model was used to analyze the association between baseline BMI and lung cancer in non-smoking males. Results: By December 31, 2011, a total of 48 799 male non-smokers had been assessed in the study and there were 214 620.18 person-years of follow-up and 4.40 years of average follow-up period. During follow-up, 198 lung cancer cases were identified among the 48 799 non-smoking males. Compared with those with normal BMI (kg/m2) (18.5≤BMI<24.0), the hazard ratio (95%CI) of lung cancer were 1.14(0.53-2.45), 0.57(0.41-0.78) and 0.61(0.38-0.97) for underweight (BMI<18.5), overweight (24.0≤BMI<28.0) and obese males (≥28.0), respectively, after adjustment for potential confounding factors, including age, education level, alcohol consumption, physical activity, history of diabetes and work environment. Baseline BMI was negatively associated with the risk of lung cancer in non-smoking males and the risk would be reduced by 22% (HR=0.78, 95% CI: 0.64-0.95) for per 5 kg/m2 BMI increase. The negative association between BMI and risk of lung cancer was significant among non-smoking males who aged ≥50 years, and in those who had physical exercise <4 times/week, never drunk and worked above the coal mine, and for per 5 kg/m2 BMI increase, the risk of lung cancer would be reduced by 26%(HR=0.74, 95%CI: 0.60-0.92), 24%(HR=0.76, 95%CI: 0.62-0.95), 20% (HR=0.80, 95%CI: 0.65-1.00) and 23% (HR=0.77, 95%CI: 0.61-0.97), respectively. The result was similar after excluding the cancer patients newly diagnose within 1-year and their contribution person years. Conclusion: Baseline BMI might be negatively associated with risk of lung cancer in non-smoking males.