Establishment of scientificaly sound criteria for breaking into stages the intellectual development of children is stated as a specific problem. It is assumed that stages of intellectual development can be properly diagnosed only in a formative experiment. Two equivalent indicators of the intellectual stage achieved by a given child are being theoretically introduced and experimentally proved by the authors: 1) the highest level of thinking where the child can for the first time perform a new action after having passed through the stage of compiling the orientational scheme for the action, and 2) the highest level of thinking at which it is possible to convey to the child the orientational scheme for the new action which is being developed.