Triptolide, a traditional Chinese medicine, has been reported to be effective in the treatment of auto-immune diseases, and it can also induce anti-neoplastic activity on several human tumor cell lines. This study investigates the cytotoxic function and the functional mechanism of triptolide on tumor cells. Promyelocytic leukemia, (HL-60), T cell lymphoma (Jurkat), and human hepatocelluar carcinoma (SMMC-7721) cells were subjected to triptolide treatment, and cell growth inhibition was examined by XTT cell viability assay. Cell death mechanism (apoptosis) was confirmed through DNA fragmentation and DAPI staining. Triptolide inhibited 50% of cell growth (IC(50)) on HL-60 cells at 7.5 nM, Jurkat cells at 27.5 nM and SMMC cells at 32 nM. Characteristic apoptotic features including internucleosomal DNA fragmentation and chromatin condensation were observed in triptolide treated cells. Data from the study indicates that triptolide could induce apoptosis in human tumor cell lines and it may be applicable as a potential chemotherapeutic agent for cancer treatment.