1,4-Naphthoquinones are widely distributed in nature and many clinically important antitumor drugs containing a quinone moiety, such as anthracyclines, mitoxantrones and saintopin, show excellent anticancer activity. In this study, 2- or 6-substituted 5,8-dimethoxy-1,4-naphthoquinone (DMNQ) and 5,8-dihydroxy-1,4-naphthoquinone (DHNQ) derivatives were synthesized, and their cytotoxic activity against L1210 and P388 cancer cells was examined. Their antitumor activity was also assessed in mice bearing S-180 cells in the peritoneal cavity. In comparison with the DMNQ derivatives, the DHNQ derivatives exhibited more potent bioactivities than the DMNQ derivatives against both L1210 and P388 cells in vitro and S-180 cells in vivo. The $ED_{50}$ values of the DHNQ derivatives against P388 cells were in the range of 0.18-1.81 ${mu}g/mL$ whereas those of the DMNQ derivatives were in the range of 0.26-40.41 ${mu}g/mL$. The T/C ($\%$) values of the DHNQ derivatives, 8, 17, 18, 19, and 20, were found to be comparable to or even better than that of adriamycin. It was also observed that the 2-substituted derivatives (8, 19, 20) showed better antitumor activity than the 6-substituted derivatives (7, 17, 18) in the mice bearing S-180 cells in the peritoneal cavity.