Objective : To investigate the effects of branched-chain amino acid(BCAA) on the differentiation of 3T3-L1 preadipocytes and its potential mechanism. Methods : 3T3-L1 preadipocytes were divided into the Control, differentiation medium(DM), low-concentration BCAA, and high-concentration BCAA groups. A CCK-8 assay was utilized to evaluate pre-adipocyte survival under various BCAA concentrations. Oil-red O staining was used to observe the formation of lipid droplets in adipocytes. Intracellular triglyceride(TG) and total cholesterol(TC) were detected by enzymatic method. RT-qPCR and Western blot were used to detect the mRNA and protein expression of Stat3 and adipocyte differentiation-related genes. Results : CCK-8 results showed that the viability of 3T3-L1 cells was not affected when the BCAA concentration was ≤ 10 mmol/L. Compared with the DM group, the low-concentration BCAA groups(0.5 and 1.0 mmol/L) had significantly larger intracellular lipid droplets, increased number of lipid droplets, and elevated levels of the intracellular TC(0.88vs0.68 mmol/g; 0.83vs0.68 mmol/g,P<0.01) and TG(0.77vs0.40 mmol/g; 0.62vs0.40 mmol/g,P<0.01). Nevertheless, the cell differentiation in the high-concentration group(5.0 and 10.0 mmol/L) significantly decreased compared with that in the DM group. Further, levels of PPARγ, C/EBPα, Adiponectin, and FABP4 mRNA and protein expression significantly increased in the low-concentration group, but significantly decreased in the high-concentration group than that in the DM group(P<0.01). In addition, low concentrations of BCAA promoted stat3 phosphorylation, while high concentrations inhibited its phosphorylation(P<0.01). Conclusion BCAA have a dual role in regulating the differentiation of preadipocytes through Stat3, i.e. low concentrations of BCAA induce cell differentiation by promoting Stat3 phosphorylation; whereas high concentrations of BCAA inhibit Stat3 phosphorylation and cell differentiation.