The antitumor effect of hydrogen nanobubble was evaluated in vitro and in vivo. Cancer cell growth was inhibited in the hydrogen nanobubble-containing medium compared to the non-containing medium (in vitro). Tumor growth was significantly suppressed in the middle stage of transplantation (in vivo). We reported that hydrogen nanobubbles is a ROS scavenger, therefore apoptosis due to ROS reduction may be induced in these results.