Rationale: It remains unknown whether long-term ozone exposure can impair lung function. Objectives: To investigate the associations between long-term ozone exposure and adult lung function in China. Methods: Lung function results and diagnosis of small airway dysfunction (SAD) were collected from a cross-sectional study, the China Pulmonary Health Study (N= 50,991). We used multivariable linear and logistic regression models to examine the associations of long-term ozone exposure with lung function parameters and SAD, respectively, adjusting for demographic characteristics, individual risk factors, and longitudinal trends. We then performed a stratification analysis by chronic obstructive pulmonary disease (COPD). Measurements and Main Results: We observed that each 1 SD (4.9 ppb) increase in warm-season ozone concentrations was associated with a 14.2 ml/s (95% confidence interval [CI], 8.8-19.6 ml/s] decrease in forced expiratory flow at the 75th percentile of vital capacity and a 29.5 ml/s (95% CI, 19.6-39.5 ml/s) decrease in mean forced expiratory flow between the 25th and 75th percentile of vital capacity. The odds ratio of SAD was 1.09 (95% CI, 1.06-1.11) for a 1 SD increase in warm-season ozone concentrations. Meanwhile, we observed a significant association with decreased FEV1/FVC but not with FEV1 or FVC. The association estimates were greater in the COPD group than in the non-COPD group. Conclusions: We found independent associations of long-term ozone exposure with impaired small airway function and higher SAD risks, while the associations with airflow obstruction were weak. Patients with COPD appear to be more vulnerable.