The purpose of this study is to determine developmental process of self-concept in 6-12 years-old Iranian children and its relationships with sex and socio-economic status. Two hundred and sixty three children (120 girls and 143 boys) in four age groups (6, 8, 10 & 12yrs.) were randomly selected. Method of this study was clinical interview. Data were coded on the Damon and Hart’s Scale. Data were anlysed by MANOVA and Tuky (HSD). The results demonstrated that, first; Iranian 6yrs. children were in level 2 of developmental stage(comparative assessment level); 8yrs. were in level 3 (inter-personal implication level); 10& 12 yrs. were in level 3 too, however, less than one third of children in these groups were in level 4 (systematic beliefs and plans level). Second; the socio-economic status influenced the development of self-concept, but sex variable didn’t. Third, subjects in higher socio-economic status were in higher levels of self-concept. In addition; the first description of the self in children was physical and then by increasing the age, active, social and psychological self description were also increased.