Regulative function of preschoolers' self-estimation and conditions of its development in play activity have been experimentally studied. Effect of an expected group estimation on self-estimation comprises in senior preschoolers a psychological mechanism responsible for a trend on their part to demand principal roles in real playing situations as compared with imagined ones. Appearance of such dimensions of self-estimation as reflexivity and realism correlates with the level of correspondence of child's self-estimation to his estimation by the group. This in turn requires better organization of the play and higher level of communication in play.