The relationship among food behavior, psychological factor(life strees) and perceive tiredness was investigated in 514 adolescents. Tiredness was measured in four cases: in lessons, before school, in the early evening and at weekends. Adolescents felt most tired in lessons and before school, and leat at weekends. Irregularity of meal times and skipping breakfast were positively correlated with tiredness in lessons and befor school. Consumption of some foods was related to perceived tiredness. High sugar or fat containing ice-cream, hamburger of fizzy drinks in boys, and crisps and cookies in girls were positively associated with tiredness. Whereas beans and mushrooms were negatively associated with tiredness in girls. Life stress(by scholastic problem and personal problem) of adolescents was positively related to tiredness. The contribution of stress to tiredness was much greater than of food behavior according to multiple linear regression .There was no relationship between nutrients intake and tiredness in lessons and before school which were more strongly influenced by stress, whereas the relationship was found at weekend without influences of stress. The more tired, the lower NARs(nutrient adequacy ratio) of energy, phosphorus, vitamin A, vitamin B$_2$, and niacin were. In conclusion, some eating behavior and psychological factor can cause tiredness in adolescents, but tiredness in school and before school had more of a psychological cause than at weekend. (Korean J Nutrition 34(1):89-97, 2001)