Inositol hexaphosphate (IP₆) is a major constituent of most cereals, legumes, nuts, oil seeds and soybean. Previous studies reported the anticancer effect of IP₆ and suggested that co-treatment of IP₆ with inositol may enhance anticancer effect of IP₆. Although the anticancer effect of IP₆ has been intensively studied, the combinational effect of IP₆ and inositol and involved mechanisms are not well understood so far. In the present study, we investigated the effect of IP₆ and myo -inositol (MI) on cell cycle regulation and apoptosis using PC3 prostate cancer cell lines. When cells were co-treated with IP₆ and MI, the extent of cell growth inhibition was significantly increased than that by IP₆ alone. To identify the effect of IP₆ and MI on apoptosis, the activity of caspase-3 was measured. The caspase-3 activity was significantly increased when cells were treated with either IP₆ alone or both IP₆ and MI, with no significant enhancement by co-treatment. To investigate the effect of IP₆ and MI of cell cycle arrest, we measured p21 mRNA expression in PC3 cells and observed significant increase in p21 mRNA by IP₆. But synergistic regulation by co-treatment with IP₆ and MI was not observed. In addition, there was no significant effect by co-treatment compared to IP₆ treatment on the regulation of cell cycle progression although IP₆ significantly changed cell cycle distribution in the presence of MI or not. Therefore, these findings support that IP₆ has anticancer function by induction of apoptosis and regulation of cell cycle. However, synergistic effect by MI on cell cycle regulation and apoptosis was not observed in PC3 prostate cancer cells.