Objective To investigate the expression of miR-155 in rats exposed to PM2.5 and its signifi-cance.Methods Thirty-two 12-weeks-old healthy male SD rats were randomly divided into PM2.5-primary expo-sure group (P1),PM2.5-three times exposure group (P3), saline-control group and blank-control group (O), with 8 rats in each group. The pathological changes of lung tissue of each group were observed by HE staining and the expressions of miR-155 in lung tissue of each group were measured by RT-PCR. The expressions of TNF-α, IL-6 and IL-1β in serum of each group were measured by ELISA.One-way ANOVA and LSD-t test were used for statistical analysis,and Pearson′s correlation analysis was used to evaluate the relationship between the levels of miR-155 and TNF-α,IL-6 and IL-1β. Results The expression of inflammatory cytokines TNF-α,IL-6 and IL-1β in P1 and P3 increased when compared with that in saline control group and blank-control group, and the amount of expression increased with the increase of the number of exposure and dose;the difference between groups was statistically sig-nificant(P<0.01).At the same time,with the increase of the dose and number of PM2.5,the inflammatory damage of lung tissue was enhanced.The expression of miR-155 in lung tissue also increased with the increase of the dose and number of PM 2.5,and it had significantly positive correlation with serum inflammatory cytokines(P<0.01),and had statistical significance. Conclusions The lung tissue of rats exposed to PM2.5 can produce inflammatory injury, and the damage is enhanced with the increase of exposure times and dose.The expression of miR-155 is positively correlated with inflammatory injury in rats.Consequently,miR-155 may participate in PM2.5-induced inflammatory lung injury in rats.