Group socialization theory is the theory focusing on effect of peers on personality development. The theory asserts that the processes of assimilation and differentiation within peer groups have a crucial effect on personality development. However, the degree to which personality development is affected by such assimilation and differentiation is neglected, since many existing researches manipulate assimilation and differentiation through experimental methods. In order to test the impact of peers on personality development, a scale was developed to assess within-group assimilation and differentiation in the context of extracurricular activities. Analyses partially supported the reliability and validity of the scale.