The risk of committing a crime, including drug offenses, increases in adolescents. Along with the increase in drug offenses, the majority of those in medical reformatories for juvenile offenders are youth8 with mental illness, rather than those with physical illness. A decline in executive function is one of the commonly reported neuropsychological traits of substance dependents; however. most studies examine substance dependence in adults. and there have been only a small number of studies on adolescent substance abuse. limiting the type of substance to marijuana and alcohol. ln this study. the executive function, personality. and behavior in male youths of a medical juvenile reformatory, whose rate of substance abuse offenders is higher than that in general. were investigated. The results demonstrated higher degrees of impulsivity, verbal aggression, and apathy in youths with substance dependence than in those with an equivalent lQ and criminal history without experience of illegal drug use. A decline in the Wisconsin Card Sorting Test results Was also observed in the former. On the other hand, per80nality and behavioural traits, such as enhanced apathy, were observed in youth8 with experience of illegal drug use without having a diagnosis of substance dependence. These results suggest that it is important for health care professionals to provide medical treatment and support while fully understanding the personality and behavioural tendencies, peculiar to adolescent substance abuse, such as high degree8 of impulsivity. verbal aggression, and apathy.