Objective:To investigate the effects of ursolic acid (UA) on oxidative stress and inflammatory factors in a rat model of AR after PM2.5 exposure.Methods:Sixty healthy female SD rats were randomly divided into five groups: normal control group (NC group), PM2.5 unexposed AR group (AR group), PM2.5 exposed AR group (ARE group), UA intervention AR group (AR+UA group), and UA intervention PM2.5 exposed AR group (ARE+UA group), with 12 rats in each group. AR model was performed by a basal sensitization with intraperitoneal injection of ovalbumin (OVA) and followed by nasal instillation. PM2.5 exposure was carried out by inhalation exposure system at a concentration of 200 μg/m 3 for 3 h/d for 30 days. UA intervention group was given UA intragastric administration at 20 mg/(kg·d). AR symptoms including sneezing, nasal scratching and nasal secretion of rats in each group were observed. The activities of superoxide dismutase (SOD) and the level of malondialdehyde (MDA) in nasal mucosa were tested. The pathological changes of nasal mucosa were observed by HE staining. The levels of OVA-sIgE, IL-6 and IL-17 in serum were measured by enzyme-linked immunosorbent assay (ELISA). Protein microarray was used to measure the expression of multiple inflammation cell factors in nasal mucosa. Statistical analysis was performed with SPSS 20.0. Results:After UA intervention, the frequency of nasal sneezing, scratching and nasal secretion in ARE+UA group were lower than those of ARE group ( P<0.05). Pathological examination of nasal mucosa showed that ARE+UA group had less inflammatory granulocyte infiltration and less pathological damage to the epithelial layer than ARE group. The activities of SOD in nasal mucosa of ARE+UA group were higher than those of ARE group ((50.10±3.09) U/mg vs (20.13±1.30) U/mg, F value was 597.54, P<0.01). The contents of MDA in nasal mucosa of ARE+UA group were lower than those of ARE group ((57.78±12.36) nmol/g vs (124.12±9.40) nmol/g, F value was 115.51, P<0.01). The expression levels of OVA-sIgE, IL-6 and IL-17 proteins were lower in the ARE+UA group than those in ARE group ((11.61±0.27) ng/ml vs (20.30±0.67) ng/ml, (47.59±15.49) pg/ml vs (98.83±10.98) pg/ml, (623.30±8.75) pg/ml vs (913.32±9.06) pg/ml, F value was 283.42, 80.45, 683.73, respectively, all P<0.01). After UA intervention, protein microarray analysis showed that the expression of IL-4, IL-6, IL-13, chemokine CXCL7, IL-1α, IL-1β, MMP-8 and MCP-1 in ARE+UA group was decreased compared with ARE group while IFN-γ and IL-10 increased (all P<0.01). Conclusion:UA can reduce the aggravated AR symptoms and pathological damage of nasal mucosa, inhibit oxidative stress and release of inflammatory factors after PM2.5 exposure, and thus plays a protective role in the pathological damage of AR induced by PM2.5 exposure.