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.

The aim of experimental study is testing of the possibility and efficiency of conservation understanding formation in preschool children through the formation of their logical operations of classification and seriation. New training methods of classification and seriation were developed. They are based on the mastery of the actions belonging to the composition and structure of the data logic operations. Parallel formation of logical operations of classification and seriation using these new techniques allowed the subject not only to master these logic operations in a full, but also let the children overcome nonconservation phenomena. The possibility of formation of some concrete-operational structures of intelligence (understanding of the conservation principle) through other specific-operational structures of intelligence (the composition and structure of logical operations of classification and seriation) was proved for the first time.

In the study the levels decentration development obtained by Hughes and Piaget’s methods are compared both among themselves and with the levels of formation of classification, seriation logical operations and conservation principle understanding at preschool children. Statistically significant correlation interdependence (r-Spearman) between all investigated indicators is proved. The Piaget’s position on the importance of cognitive egocentrism as the starting point of child development, determined the logic thinking characteristics of the child was substantiated, it is necessary to consider all characteristics of logic thinking by designing and evaluating of decentration diagnostic methods. It is concluded that the Hughes and Piaget’s decentration diagnostic methods should be viewed as complementary rather than mutually exclusive when assessing the overall level of development of logical thinking of the child.