OBJECTIVE To investigate the effect of berberine on differentiation of rat bone marrow mesenchymal stem cells (MSCs) to adipocytes and its mechanism. METHODS Rat MSCs were isolated and cultured, adipocytic differentiation was induced with adipogenesis-inducing medium (AIM). Cells were assigned into 6 groups:normal control, AIM group, AIM+berberine 0.1, 0.3, 1 and 3 μmol·L-1 groups, respectively. Morphology characteristics of mesenchymal stem cells were observed under an inverted microscope and adipocyte levels were analyzed by oil O staining. Alkaline phosphatase (ALP) activity was detected using p-nitrophenyl phosphate as a substrate. The cell survival was determined by MTT assay. Expressions of peroxisome proliferator activated receptor γ (PPARγ), fatty acid binding protein (aP2) and CCAAT enhancer-binding protein α (C/EBPα) mRNA were detected by semiquantitative RT-PCR. RESULTS Compared with normal control group, MSCs adipogenic differentiation, PPARγ, aP2 and C/EBPα mRNA expression significantly increased in AIM group (P<0.01), ALP activity in AIM group significantly decreased (P<0.01). Compared with AIM group, berberine inhibited MSCs adipogenic differentiation (P<0.01) and berberine 0.1, 0.3, 1 and 3 μmol·L-1 increased ALP activity by 26%, 54%, 81% and 122%, respectively. Berberine 3 μmol·L-1 significantly downregulated PPARγ expression (0.91±0.10 vs 1.34±0.06) (P<0.01), aP2 (1.05±0.10 vs 1.53±0.09) (P<0.01) and C/EBPα mRNA (1.24±0.06 vs 1.54±0.09) (P<0.01). Berberine had no effect on proliferation of MSCs. CONCLUSION Berberine inhibits differentiation of MSCs into adipocytes, which might be closely related to the downregulation of PPARγ, aP2 and C/EBPα mRNA.