1,6-Bis[4-(4-amino-3-hydroxyphenoxy)phenyl]diamantane (DPD) induces growth inhibition in human cancer cells. In our previous study, we discovered that DPD irreversibly inhibits the growth of Colo 205 colon cancer cells at the G0/G1 phase and induces cell differentiation. However, the detailed mechanism is still unknown. In this study, we examined the functional importance of p21 and p53 in DPD-induced anticancer effects. We used three isogenic cell lines, HCT-116, HCT-116 p53-/- and HCT-116 p21-/-, to evaluate the roles of p21 and p53 in the in vitro anticancer effects of DPD. The in vivo anti-proliferative effect of DPD was demonstrated by HCT-116 and HCT-116 p21-/- xenograft models. DPD significantly inhibited the growth as well as increased the number of HCT-116 cells in the G0/G1 phase, but not in HCT-116 p53-/- and HCT-116 p21-/- cells examined by flow cytometry. Additionally, western blot analysis showed that DPD treatment induced p21, but not p53 protein expression in HCT-116 cells. The p21-associated cell cycle regulated proteins, such as cyclin D, CDK4 and pRb were decreased after DPD treatment in HCT-116 cells. The DPD-increased G0/G1 phase and induced cell cycle regulated protein expression were not observed in HCT-116 p21-/- and HCT-116 p53-/- cells. DPD decreased cell migration in HCT-116 and HCT-116 p53-/- but not in HCT-116 p21-/- cells. p21 was required for the DPD-induced in vitro anti-colon cancer effect. The in vivo study also showed that DPD significantly inhibited tumor growth through p21 signaling. Our results clearly demonstrate that DPD-induced in vitro and in vivo anticancer effects through the activation of p21 in HCT-116 cells.