While evidence supporting for nonadditive genetic influences on personality traits in Caucasian populations has been growing in recent years, twin studies that explored the existence of genetic nonadditivity in personality variation in Asian populations are still lacking. Seven hundred and sixty-five pairs of adolescent and young adult twins registered with the South Korean Twin Registry completed the 7 scales of the Eysenck Personality Scales through a mail survey. Maximum likelihood twin correlations were computed and model-fitting analyses were conducted. Monozygotic twin correlations were consistently higher than twice the dizygotic twin correlations for all 7 scales, suggesting pervasive influences of nonadditive genetic effects on personality traits in the South Korean population. Model-fitting analyses indicated that genetic nonadditivity is particularly important for the variation of Impulsivity, Venturesomeness, Empathy, Lie, and Psychoticism. According to the best fitting models, nonadditive genetic effects ranged from 34 to 49% for these scales. For Neuroticism and Extraversion, models that included an additive genetic component fit better than those including a nonadditive genetic variance component.