TGF-β1, upregulated in keloid tissue, promotes the proliferation, collagen formation and differentiation of dermal fibroblasts. miR-21 is one of microRNAs first found in human genome. The aim of our study is to explore the mechanisms of miR-21 in TGF-β1-induced scar fibroblasts proliferation and transdifferentiation. In the present study, first we found that TGF-β1 promoted scar fibroblasts proliferation and transdifferentiation via up-regulating miR-21 expression, which could be attenuated when miR-21 was inhibited. Overexpression of miR-21 had similar effect as TGF-β1 on proliferation and transdifferentiation. Additionally, TGF-β1 increased the expressions and activities of MMP2 and MMP9 in keloid fibroblasts, which was suppressed by miR-21 inhibition. Finally, the results demonstrated that PTEN/AKT signaling pathway played important role in TGF-β1-induced transdifferentiation. In conclusion, our study suggests that TGF-β1 promotes keloid fibroblasts proliferation and transdifferentiation via up-regulation of miR-21 and PTEN/AKT signalling pathway plays important role in this process, which provides a potential theoretical basis for clinical treatment of skin scars.