Objective：To investigate the alleviation of inflammatory effects and mechanisms of Streptococcus mitis in infection with Pseudomonas aeruginosa. Methods：Wild-type C57BL/6 and TLR2-/- mice were randomly assigned into 4 groups（20 in each group）. They were treated with an intratracheal instilla－tion of Pseudomonas aeruginosa（PAO1 group），Streptococcus mitis（S.mitis group），Pseudomonas aeruginosa and Streptococcus mitis（PAO1+S.mitis group），and phosphate-buffered saline（PBS group），respectively. The mice were sacrificed at 48 h after instillation；the pathological changes in lung tissues were ex－amined with hematoxylin-eosin（HE）；the levels of IL-6 and TNF-α in broncho-alveolar lavage fluid（BALF） were evaluated by ELISA；the total cell counts and neutrophil counts in the BALF were quantified. Results：In wild-type C57BL/6 mice，the bronchi and blood vessels in PAO1 group was infiltrated by a large number of inflammatory cells and was complicated with alveolar septum thickening. The pathological changes were milder in PAO1+S.mitis group than in PAO1 group，while there was no significant pathological changes in lung tissue in PBS group and S.mitis group. The levels of IL-6，TNF-α were significantly higher in PAO1 group than that in PAO1+S.mitis group（P=0.032，P=0.007）. The total number of BALF cells and neutrophils in PAO1 group was the highest among the four groups（P＜0.05）. In TLR2-/- mice，alveolar septum thickening and the infiltration of inflammatory cells was ob－served in PAO1 group and PAO1+S.mitis group. No statistically significant difference in the expression of IL-6，TNF-α，total number of BALF cells and neutrophils in PAO1 group and PAO1+S.mitis group（all P=1.000） was observed. Conclusion：Streptococcus mitis can alleviate the inflammatory response of Pseudomonas aeruginosa and is related to TLR2.