During the last two decades etiological studies have revealed that gastric cancer is associated with Helicobacter pylori (H. pylori) infection, cigarette smoking, unbalanced dietary habit, alcohol abuse, and pathophysiological stress. These conditions may be closely related to gastric cancer through the production of reactive oxygen species (ROS) such as hydroxyl free radical, and reactive nitrogen species (RNS) such as nitric oxide. Moreover a potent chemical gastric carcinogen, 1-methyl-3-nitro-1-nitrosoguanidine (MNNG), is responsible for the carcinogenesis via formation of ROS and RNS.  In this review we show some evidence from which a theory that ROS and RNS can be potent carcinogens for gastric mucosa is derived.