The purpose of this study was to evaluate the reliability and validity of the Junior Temperament and Character Inventory (Junior TCI), a child and adolescent version of the Adult Temperament and Character Inventory (TCI), which is based on Cloninger's biosocial model of personality. The Junior TCI was translated into Korean and administered to 663 Korean middle school students (male/female, 360/303; age, 13.3 ± 1.0 years) and their parents (mother/father, 444/84). Internal consistency was calculated by Cronbach α. A test-retest study of the Junior TCI was conducted across a 3-month interval with 97 subjects. Factor analyses for the temperament and character dimensions were performed using principal component analysis, rotating factors by Promax. Cronbach α values of the Junior TCI scales ranged from .48 to .80 for the temperament scales and from .64 to .68 for the character scales. Test-retest correlations ranged from .62 to .85 for the temperament scales and from .76 to .79 for the character scales. Principal component factor analyses showed three factors out of four temperament scales (Harm Avoidance 1 to 4; Novelty Seeking 1 to 4; Persistence; and Reward Dependence 1, 3, 4) and three factors out of three character scales (Self-Directedness 1 to 5; Cooperativeness 1 to 5; and Self-Transcendence 1 to 3) that were similar to factor structures of adult versions of the TCI. Explorative factor analysis with a condition of eigenvalue greater than 1 produced six factors, as compared to seven factors extracted in Cloninger's original report of the adult TCI. In conclusion, this was the first study to tests and report detailed psychometric properties of the Junior TCI. The current study confirms that the Junior TCI has satisfactory psychometric properties for the use in child and adolescent populations and reflects Cloninger's biosocial model of personality.