Purpose: This study aimed to investigate the effect of smoking duration, smoked cigarettes per day and smoking cessation period on pulmonary function among ex-smokers: based on the 6th Korea National Health and Nutrition Examination Survey (KNHANES). Methods: This study was analyzed using the 6th KNHANES data. Pulmonary function tests were performed on a total of 4,214 adults (>40 years old). A total of 770 adults ex-smokers were eligible for inclusion in the final analysis. Forced vital capacity (FVC), forced expiratory volume in one second (FEV1) and FEV1/FEV were measured to evaluate pulmonary function. Results: This study showed that there were significant differences in both FEV1 and FVC values based on gender, age and height, among ex-smokers. FEV1/FVC significantly differed by age, height and the smoking duration prior to smoking cessation. Multiple regression analysis revealed that, FEV1/FVC accounted for 26.0% of the variance by age, height and smoking duration. There was a difference in the mean value of FEV1/FVC with or without smoking for more than 10 years. Conclusion: This study’s findings show that smoking for over 10 years in an ex-smoker can lead to problems with the respiratory system. The long-term cigarette has progressive ill effects on the respiratory system.