Inhibition of the STAT3 signaling pathway is involved in the antitumor activity of cepharanthine in SaOS2 cells. To investigate the molecular mechanisms underlying the antitumor activity of cepharanthine (CEP), an alkaloid extracted from Stephania cepharantha Hayata.Human osteosarcoma cell line SaOS2 was used. MTT assay, Hoechst 33342 nuclear staining, flow cytometry, Western blotting and nude mouse xenografts of SaOS2 cells were applied to examine the antitumor activity of CEP in vitro and in vivo. The expression levels of STAT3 and its downstream signaling molecules were measured with Western blotting and immunochemistry analysis. The activity of STAT3 was detected based on the phosphorylation level of STAT3, luciferase gene reporter assay and translocation of STAT3 to the nucleus.Treatment of SaOS2 cells with CEP (2.5-20 μmol/L) inhibited the cell growth in a concentration- and time-dependent manner. CEP (10 μmol/L) caused cell cycle arrest at G(1) phase and induced apoptosis of SaOS2 cells. CEP (10 and 15 μmol/L) significantly decreased the expression of STAT3 in SaOS2 cells. Furthermore, CEP (5 and 10 μmol/L) significantly inhibited the expression of target genes of STAT3, including the anti-apoptotic gene Bcl-xL and the cell cycle regulators c-Myc and cyclin D1. In nude mouse xenografts of SaOS2 cells, CEP (20 mg·kg(-1)·d(-1), ip for 19 d) significantly reduced the volume and weight of the tumor.Our findings suggest that inhibition of STAT3 signaling pathway is involved in the anti-tumor activity of CEP.