Previous clinical studies have shown efficacy and safety of the traditional Chinese medicinal herb extract Chan-Yu-Bao-Yuan-Tang (CYBYT) in lung cancer patients. The effects of CYBYT on proliferation and apoptosis in the non-small cell lung cancer cell line NCI-H460 and the small cell lung cancer cell line NCI-H446 were investigated in vitro. CYBYT significantly induced antiproliferative effects of NCI-H460 and NCI-H446 cells in a concentration- and time-dependent manner. The IC(50) values in NCI-H460 cells were 94.37 µg/mL (24 h) and 20.89 µg/mL (48 h), whereas in NCI-H446 cells IC(50) values were 214.72 µg/mL (24 h) and 114.58 µg/mL (48 h). Annexin V-FITC/PI staining showed CYBYT could significantly induce apoptosis in NCI-H460 and NCI-H446 cells, and the total apoptosis rates were positively correlated with the concentration and time of CYBYT treatment. Furthermore, treatment with the broad-spectrum caspase inhibitor (z-VAD-fmk) effectively protected NCI-H460 and NCI-H446 cells against CYBYT-triggered apoptosis. The apoptotic processes involved were a marked decrease in antiapoptotic protein Bcl-2 and an increase in proapoptotic protein Bax. The release of mitochondrial cytochrome c into the cytosol was also observed, which, in turn, resulted in the activation of caspase-9 and caspase-3. CYBYT exerts antiproliferative and growth inhibition effects on NCI-H460 and NCI-H446 cells through the mitochondrial caspase-dependent cell death pathway.