Rhizomes of Alpinia galanga (Linn.) or ‘Kha’ in Thai are used in food and as folk medicine in South andSoutheast Asia. The aims of this study were to identify the mechanism of cell death of human leukemic HL-60 and U937 cells induced by 4’-hydroxycinnamaldehyde (4’-HCA) isolated from A. galanga. 4’-HCA wascytotoxic to both cell lines in a dose-dependent manner (p<0.05) as demonstrated by MTT assay. Apoptosisinduced by 4’-HCA was demonstrated by a variety of methods: visualization of propidium iodide (PI)-stainedcells under fluorescence microscope, detection of subdiploid cells by PI-staining and flow cytometry, and assayof active caspase-3 using a specific fluorogenic substrate. 4’-HCA-treated cells (10 and 50 μg/ml for 4 h) showedsignificant increase in reactive oxygen species production and decreased mitochondrial transmembrane potentialas detected by dichlorohydrofluorescein diacetate and 3,3’-dihexyloxacarbocyanine iodide respectively, togetherwith flow cytometry. The apoptotic death involved cytochrome c release, increase in Bax level and concomitantdecreases in levels of Bcl-2 and Bcl-xL (using Western blotting), and elevation in cytosolic and mitochondrialCa2+ contents (using compartment-specific fluorescent Ca2+ dyes). These results indicate that 4’-HCA inducesapoptosis of human leukemic cell through a combination of mitochondrial and ER stress pathways.