(a) The purpose of this study was to explore the self-control system of small groups, especially the system in which group members perceive the probability of attainment of group goal and evaluation of achievement level, by means of feedforward and feedback information. (b) Opeators in a group system were based on various relations expressed as the difference equations of the input and output of the operators, and weights of variables in equations were estimated by regression coefficients. (c) The experimental work of groups was to add from one to four to random numbers. Subjects were fifth grade boys. In the experimental situation, group goal and group performance were informed to the group members. In the control situation, group goal was informed but the group performance was not. A maximizing task and optimizing task were used. (d) Results showed that feedforward and feedback information were frequently used to perceive the probability of attainment of group goal and evaluate the level of achievement.