Stromal fibroblasts are located in the surrounding of parenchyma cells and involved in secretion of extracellular matrix (ECM). The cells are quiescent in general state, but abnormally activated when diseases such as inflammation and tumors occur. The activation of fibroblasts is regulated by various genes, which can promote cell proliferation and secretion of ECM like collagen and fibrin, thus leading to disease progression induced by excessive local fibrosis. Yes-associated protein 1 (YAP1) is not only a coactivator of transcription factor, but also an effector in multiple signaling pathways. The expression level of YAP1 is up-regulated in activated fibroblast. Moreover, the protein could contribute to activation of fibroblasts through regulating the expression of activation-related genes like growth factors. As a result, YAP1 may be a key regulator in fibroblast activation, and inhibition of YAP is expected to be a potential treatment of fibrosis.