Abstract: Objective: To investigate the expression of miR-155 in peripheral blood and tissues of oral lichen planus patients. Methods: Eighty-five oral lichen planus patients were selected. They were divided into non-erosive type (n=48) and erosive type (n=37), according to the different conditions of the base of mucosa of lesions. Forty healthy subjects and forty-five normal tissues were selected as control group. The expressions of miR-155 in plasma and mononuclear cells in patients and the control group were detected by real-time quantitative PCR. And the expressions of miR-155 in mucosa of patients and normal mucosa were also detected. The efficacy of miR-155 in predicting the extent of lesions in patients was analyzed using ROC curve. Results: The relative expression levels of miR-155 in plasma and mononuclear cells of oral lichen planus patients (1.47±0.34) were significantly higher than the control group (0.95±0.12) (P<0.05). The relative expressions of miR-155 in plasma and tissue of erosive type group were higher than the non-erosive type group (P<0.05). ROC curve analysis showed that the relative expression of miR-155 in plasma of oral lichen planus patients in predicting the extent of the lesion was 0.807 (95%CI: 0.712-0.903). The sensitivity was 67.6% and specificity was 85.4%. The relative expression of miR-155 in tissues of oral lichen planus patients in predicting the extent of the lesion was 0.845 (95%CI: 0.761-0.928). The sensitivity was 70.3% and specificity was 87.5%. Conclusion: miR-155 might be used as a predictor index for the severity of the patient's lesions.