Objective：This study aimed to investigate the promotion effect of low molecular weight hyaluronic acid（LMW-HA）on phenotypic transformation of mice cardiac fibroblasts and the role of CD44 and S100A4 in this process. Methods：Cardiac fibroblasts（CFs）were isolated and cultured from neonatal ICR miceandthen stimulated by LMW-HA. The proliferation of CFs was measured by CCK-8 and EdU. Western blot（WB），quantitative RT-PCR and immunofluorescence were used to determine myocardial fibrosis. Nuclear and cytoplasmic proteins of CD44 and S100A4 were extracted to specify the nuclear translocation by WB and immunofluorescence. Above indicators were measured again after CFs treated with the CD44 inhibitor BRIC-235. Results：CCK-8 and EdU determined that the optimal concentration of LMW-HA stimulation was 0.8mg/mL. WB，qRT-PCR and immunofluorescence showed that LMW-HA stimulation significantly increased the level of myocardial fibrosis markers（α-SMA and Collagen 3），but CD44 remained unchanged，and S100A4 gradually increased with time.The proteins of CD44 and S100A4 were transferred into the nucleus at the same time. BRIC-235 can inhibit these changes. Conclusion：LMW-HA can promote the differentiation of micecardiac fibroblasts，which is regulated by the nuclear translocation of CD44 and S100A4 and the activation of downstream signaling pathway.