Objective: To investigate the effect of PM2.5 exposure on the thyroid function in male SD rats. Methods: Adult Sprague-Dawley rats were divided into " filtered air" (FA) and " PM2.5" groups. These rats exposed to the 2 kinds of air for 12 weeks were killed and their plasma and thyroid tissues were taken for investigation. The morphological status and pathological changes of thyroid tissue were observed after H&E staining. The levels of triiodothyronine (T₃), free triiodothyronine (FT₃), thyroxine(T₄), free thyroxine (FT₄), anti-thyroglobulin antibody (TGAb), anti-thyroid peroxidase antibody (TPOAb), thyrotropin (TSH), tumor necrosis factor α (TNF-α), and interleukin-6 (IL-6) in rats were measured by ELISA. Results: The results showed that there was no significant difference in body weight between two groups after 12 weeks of exposure to either filtered air or PM2.5. No pathological changes and significant differences were observed in the thyroid tissues of two groups after HE staining. There were no significant differences in plasma levels of T₄, T₃, FT₃, TNF-α, and IL-6 between two groups. However, compared with rats exposed to filtered air, rats exposed to PM2.5 had significantly lower plasma FT₄[(3.81±1.23) vs (6.02±2.40)pmol/L, P<0.01] and higher TSH [(5.63±1.74) vs (4.29±1.37) mU/L, P<0.05] levels. Conclusion The exposure to PM2.5 for a period of 12 weeks may lead to the decrease of FT₄ and increase of TSH in rats.