Isoguanosine and berberine 1:1 mixture[l:[1(mole:mole)] has been prepared and evaluated by measuring antitumor effects against various tumor cell lines in culture and in mice. We reported that the synergistic effect of isoguanosine and berberine mixture has been revealed compared with each of isoguanosine and berberine increased by $3{sim}8$ times than that of each components in various tumor cell lines in vitro. The most effective dose of isoguanosine and berberine mixture was 60 mg/kg/day in mice bearing S-180 solid tumor, the %(1-T/C) values were 70%. Against the P-388 leukemia, isoguanosine-berberine mixture was the most effective at the dose of 60 mg/kg/day, the %T/C values were 163%.