Nonthermal atmospheric plasma (NTAP) can be applied to living tissues and cells as a novel technology for cancer therapy. The authors report on a NTAP argon solution generated in deionized (DI) water for treating human gastric cancer cells (NCI-N87). Our findings show that the plasma generated in DI water with 30-min duration has the strongest effect on apoptosis in precultured human gastric cancer cells. This result can be attributed to the presence of reactive oxygen species (ROS) and reactive nitrogen species (RNS) produced in water during treatment. Furthermore, the data show that the elevated levels of RNS may play a more significant role than ROS in the rate of cell death.