Triptolide, a main active component extracted from the traditional Chinese herbal medicine, Tripterygium wilfordii Hook. f (TWHf), has been shown to possess potent immunosuppressive and anti-inflammatory properties. However, the toxicity of triptolide limits its clinical applications. Here we treated the human proximal tubular epithelial cell line HK-2 cells with triptolide in vitro and investigated its toxic effects. The cytotoxicity was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for viability inhibition and annexin V/propidium iodide (PI) staining for apoptosis/necrosis. The activation of caspase 3 was analyzed by Western Blotting. MTT assay showed triptolide inhibited the viability of HK-2 cells in a time- and dose-dependent manner. Flow cytometry assay showed triptolide caused apoptosis rather than necrosis in HK-2 cells by staining with annexin V/PI. Furthermore, the increase of cleaved p17 fragment, an active form of caspase 3, was detected. These results suggested that triptolide is able to cause cytotoxicity on HK-2 cells, and the mechanism of which is associated with caspase 3.