The experimental research is directed on verifying of possibility and efficiency of formation of logic operations (classification, ranking) and understanding of a principle of conservation through formation of the decentration ability at children by the method developed by authors. Following positions are substantiated and confirmed. Formation of the decentration ability at children of the senior preschool age in the course of specially organized training influences on change of degree of intensity of an egocentrism and raises the decentration ability, influences on change of logic operations (classification, ranking) formation and understanding of a principle of conservation (length, quantity of substance and water volume) level. All changes after the formation are nonrandom.