This study was aimed to investigate the ability of a flavonoid compound breviscapine (BVP) to suppress growth and elicit apoptosis in human osteosarcoma (OS) Saos-2 cells. The cells were cultured in vitro and treated with three concentrations of BVP (80, 160, and 320 μg/ml). Moreover, C57 mice were injected with Saos-2 cells to establish a subcutaneous xenograft model, and they were subsequently treated with three doses of BVP via intraperitoneal injection. The viability of the cells was examined by the Cell Counting Kit-8 method. The apoptotic cells were assessed by flow cytometry and terminal deoxynucleotidyl transferase dUTP nick end labeling staining. The tumor volume and weight were monitored from day 3 through day 21 after the last injection. The expression of bax, bcl-2, and cytochrome c (cyt c) mRNA was detected by a real-time polymerase chain reaction. The protein levels of bax, bcl-2, cyt c, caspase 3, and caspase 9 were evaluated by Western blot. The expression and distribution of bcl-2 and bax in tissues were detected by immunohistochemistry. Compared with the control group, BVP treatment inhibited cell proliferation and induced apoptosis of Saos-2 cells in vitro. Consistently, treatment of mice bearing transplanted tumors with BVP suppressed the growth of OS tumors and promoted cell apoptosis; it also reduced tumor volume and weight. Mechanistically, BVP-induced apoptosis was mediated by the mitochondria-dependent pathway, as evidenced by the increased expression of bax and cyt c and the decreased expression of bcl-2, as well as activation of caspase 9 and caspase 3 in vitro and in vitro. Collectively, BVP inhibits growth and promotes apoptosis of OS by activating the mitochondrial apoptosis pathway. © 2020 The Authors. Journal of Biochemical and Molecular Toxicology Published by Wiley Periodicals LLC.