The main aim of this study is to estimate the heritability of AFFM and PEN dimensions, including 67 pairs of twins (34 monozygotic and 33 dizygotic) of both genders, aged 18 - 44. The heritability has been estimated by the biometric method, two full (ACE and ADE) and three reduced (AE, DE and CE) models tested for each personality trait. Taking into consideration the AFFM dimensions, additive genetic factors and a non-shared environment contribute the most significantly to the phenotypic variation of activity, sociability and the impulsive sensation seeking; anxiety and aggressiveness are best accounted for by the dominant genetic effects. In the PEN domain, fit indicators suggest that ACE and the reduced AE models provide the best explanation for the phenotypic manifestations of neuroticism, while ACE and CE models account for the variation of L scale. Although the fit indicators calculated for extraversion and psychotic behavior are somewhat problematic, the parameter estimates show that extraversion is best accounted for by the additive genetic variance, shared environmental effects, and the non-shared environment, whereas psychotic behavior is the most adequately explained by both shared and non-shared environmental effects.