Formation of identity and differentiation is crucial in the development process of adolescents and young people. Despite many studies on identity status and self-differentiation in adolescence and early adulthood stages, this issue has been less examined among twins. This study was conducted to compare self-differentiation and identity statuses in twins and nontwins. The sample of the study consisted of 128 identical twins, 176 nonidentical twins and 170 nontwins aged from 13 to 30 years and living in Mashhad in Iran, who completed the Objective Measure of Ego Identity Status and Self-Differentiation questionnaires. The results showed that the self-differentiation of identical and nonidentical twins was significantly higher than nontwins. The results also showed that level of Foreclosure identity, Moratorium identity and Achieved identity was similar among identical twins, nonidentical twins and nontwins, but significantly different in terms of Diffusion identity. Further, the results showed that twins younger than 18 years had scores higher than nontwins and twins over 18 years in terms of Diffusion identity. Findings revealed that females were significantly lower than males for Moratorium identity. The adolescence period and co-twins may be the reason for the high levels of Diffusion identity and self-differentiation. Also, it seems that identity development and differentiation are related to emotional and cognitive development.