Although neoadjuvant chemotherapy has improved the survival rate of osteosarcoma patients, drug resistance remains a predominant obstacle to improving efficacy and necessitates the development of novel chemotherapeutical agents. The aim of this study was to investigate whether tetrandrine (TET) induces apoptosis in the U-2OS and MG-63 osteosarcoma cell lines and to further determine the underlying mechanism. This study investigated the effects of TET on osteosarcoma in vitro. To examine the antitumor effects of TET on osteosarcoma, the two osteosarcoma cell lines were treated with TET and subjected to apoptosis assays. The results revealed that TET induced the apoptosis of osteosarcoma cells in a time- and dose-dependent manner. Furthermore, the apoptosis of osteosarcoma cells was accompanied by increased cytochrome c (Cyto-C), apoptotic protease-activating factor (Apaf)-1, Bid and Bax activation and reduced Bcl-2 and Bcl-xl activation, demonstrating that the apoptosis may have occurred through the mitochondrial pathway. In conclusion, the results suggest that TET is a promising agent for osteosarcoma therapy.