Abstract: The research objective is to obtain the cellular mechanism(s) of Syzygium polyanthum leaves for its development into commercial herbal product for certain degenerative diseases. The approach was to conduct activity-guided fractionation of metabolite extracts based on the antioxidant and the apoptotic modulation activities using cultured cells. Antioxidant activity of the active fractions were screened using 1,1-diphenyl-2-picrylhydrazil (DPPH) free radical assay and thiobarbituric acid (TBA) assay. The cellular effects were investigated using cultures of HB4C5 human hybridoma and mouse colon 26 adenocarcinoma cells. The flavonoids extract which has the lowest IC50 value of 14.84 μg mL-1 for DPPH assay among other metabolites, was found to have significant cytotoxic effect on both HB4C5 and colon 26 cells. Specific assays for apoptosis using caspase 3-gene expression and annexin-V flow cytometry analyses, supported the effect of cell proliferation inhibition which was not due to apoptosis. Subsequent cell cycle analysis indicated that crude extract may induce cell cycle arrest of HB4C5 cells at G1 to S phase, whereas active compound fractions of flavonoids stacked the cell cycle at G2/M phase. These observations suggest that flavonoid extracts of Syzygium polyanthum posses active compound(s) with anti-proliferative effect. This is a new finding for therapeutic potential of Syzygium polyanthum leaves in Indonesian traditional medicines.