Stabilities of five kinds of sweetening agents, aspartame, sodium saccharin, glycyrrhizinic acid, stevia type 1 and stevia type 2, were estimated in four different types of processed food samles, soda drinkings (orange and root beer), half solid food (retourt custard pudding) and powdered food (soybean sauce soup for chinese noodles). Recoveries of these agents added 500 μg/g to food samples were 92.0-101.7%. Each sweetening agent added to respective food sample at the production stagew as quantitatively analyzed by HPLC on given days thereafter, and determined the remaining ratio to the added amount. Most of the sweetening agent tested remained more than 80% even 90 days later, although the stability of the agent depended on the combination with the food sample. In the retourt custard pudding, degradation of added aspartame during the production stage of the food was suggested, because the agent could not be detected any more in the food. Similarly, stevia type 2 in the pudding was indicated to interact with some components of the food. In contrast, almost all agents were shown to be stable in the powdered food.