〔Abstract〕 Objective To investigate the role of ubiquitin carboxyl-terminal hydrolase L1(UCHL1) in the occurrence and development of myocardial fibrosis in a mouse model. Methods Male mice(n=6 per group) were continuously perfused with angiotensin II(Ang II) for 14 days to establish a mouse myocardial fibrosis model. UCHL1 specific inhibitor LDN57444 was injected intraperitoneally into these mice. Blood pressure was measured at the tail. The change in cardiac structure was evaluated by M-mode echocardiography. The severity of myocardial fibrosis was assessed after sirius red staining. The primary mouse myocardial fibroblasts were randomly divided into 3 groups: control group, P group(PDGF-DD) and PL group(PDGF-DD + LDN57444). The mRNA and protein expressions were detected by qRT-PCR and Western blot. Results Compared with the control group, the mean systolic pressure increased significantly and the cardiac structure and function changed in the animals treated with Ang II. Treatment with UCHL1 specific inhibitor LDN57444 could significantly inhibit the above Ang II-induced effects, and the severity of myocardial fibrosis decreased markedly.The mRNA expression of type I collagen in the PL group was markedly lower than that in the P group, the mRNA expression of UCH-L1 in the P and PL groups was significantly higher than that in the control. The protein expression levels of PDGFRβ/pPDGFRβ, UCH L1, pRb/Rb, and Col Ⅰ in the P group was significantly higher than those in the control group, the protein expression levels of PDGFRβ/pPDGFRβ, ColⅠ and pRb/Rb in the PL group was markedly lower than those in the P group. Chromatin immunoprecipitation assay confirmed NF-κB regulated the transcription of downstream UCH-L1. Conclusion UCH-L1 is involved in the regulation of mouse cardiac fibroblasts proliferation. In this process, the PI3 K/AKT/NF-κB pathway regulates the expression of UCH-L1 and then regulates downstream Rb protein expression, which subsequently regulates the cell cycle to affect the proliferation of cardiac fibroblasts. Inhibiting UCH-L1 can reduce myocardial fibrosis in mice.