Objective : To explore the effect of miR-21 on human periodontal ligament stem cells (PDLSCs) proliferation and osteogenesis and to provide a theoretical basis for the stem cell treatment of periodontitis. Methods: hPDLSCs were isolated and cultured with the enzymatic tissue block method, and surface molecules (CD34, CD45, CD90 and CD105) were detected by flow cytometry. An miR-21 mimics (pre-miR-21) and inhibitor (anti-miR-21) were transfected into hPDLSCs by Lipofectamine 2000. The experiment groups: mimics-NC group, mimics group, inhibitor group, and inhibitor-NC group. The transfection efficiency of miR-21 was determined by qRT-PCR. Proliferation was detected by CCK-8 and flow cytometry. The osteogenic differentiation ability of hPDLSCs was determined by alizarin red staining. Western blot was used to detect the protein expression of osteogenic related genes: Runx2. Results: The mRNA expression of miR-21in the mimics group was significantly higher than that in the mimics-NC group; additionally, the expression in the inhibitor group was significantly weaker than that in the inhibitor-NC group (P < 0.05). hPDLSCs proliferation and the S phase cell ratio in the mimics group were stronger than those in the mimics-NC group(P < 0.05); those in the inhibitor group were weaker than those in the inhibitor-NC group (P < 0.05). After alizarin red staining, the mimics group was found to have more mineralized modules than mimics-NC group, and the inhibitor group had fewer than that in the inhibitor-NC group. Runx2 protein expression in the mimics group was higher than that in the mimics-NC group (P <0.05), and expression was lower in the inhibitor group than in the inhibitor-NC group (P < 0.05). Conclusion miR-21can promote the proliferation and osteogenesic differentiation of hPDLSCs.