Aim To investigate the effect of circ_0036167 on the fibrotic phenotype of cardiac fibroblasts and the potential mechanism involved. Methods Masson's trichrome staining was performed in the myocardium of patients with heart failure (HF) and healthy organ donors. Levels of circ_0036167 and its host gene of MYO9A were determined by real-time quantitative polymerase chain reaction (RT-qPCR) in the myocardium of HF patients and healthy organ donors. Actinomycin D treatment and RNase R exonuclease digestion were used to test the stability of circ_0036167 in AC16 cells. The myocardial fibroblasts (mCF) of C57BL/6 mice were infected with recombinant circ_0036167 adenovirus, and the expression of fibrosis related genes COL1A1, COL3A1 and ACTA2 were detected at RNA and protein levels.EdU staining and transwell migration assay were performed to detect the effects of circ_0036167 on mCF proliferation and migration activities. According to the results of bioinformatic prediction, RNA binding protein immunoprecipitation (RIP) assay was performed to confirm the interaction between circ_0036167 and fused in sarcoma (FUS) protein. Effect of FUS knockdown on inhibition of fibrosis-related genes expression by circ_0036167 in mCF was determined. ResultsMasson's trichrome staining showed that the cardiac fibrosis was significantly increased in the myocardium of HF patients (P＜0.001). Circ_0036167 was found markedly increased in the myocardium of HF patients (P＜0.05), with no significant difference in its host gene of MYO9A. In response to actinomycin D treatment and RNase R exonuclease digestion, circ_0036167 was more stable than MYO9A. Over-expression of circ_0036167 suppressed proliferation and migration of mCF, and inhibited RNA and protein expression of fibrosis-related genes in mCF. RIP assay revealed the interaction between circ_0036167 and FUS protein. Knock-down of FUS could increase fibrosis-related genes expression (P＜0.05 or P＜0.01), and significantly attenuated the inhibitory effect of circ_0036167 on fibrosis-related gene expression in mCF(P＜0.01 or P＜0.001). Conclusion FUS mediates the anti-fibrotic effect of circ_0036167 in mCF.