The relations of personality characteristics, such as shyness, sociability and self-disclosure and victimization by peers to loneliness in adolescents were examined with a sample of 485 second-grade highschool students(256 boys and 229 girls). They answered the questionnaires on loneliness as well as shyness, sociability, self-disclosure, and victimization by peers. All variables were significantly related to loneliness. The more shy they were and the more victimization by peers they experienced, the more lonely they were. The more sociable they were and the more self-disclosure they showed, the less lonely they were. All variables significantly predicted loneliness and the most influential predictor of loneliness was victimization by peers, indicating that the influence of victimization by peers on loneliness is so powerful for the adolescents.