Educational activity requires such educational knowledge as ‘teaching knowledge’ for teaching activity and ‘learning knowledge’ for learning activity. Even though it has academic and practical significance, it would be hard to say that the nature of educational knowledge has been fully illuminated and discussed. But it is supposed that educational knowledge is differentiated from knowledge of subject matter. Moreover, it is a unique frame of experiencing education(teaching or learning); it is tacit, personal, and practical, and it has developmental hierarchies; each person evaluates education according to their own criteria, which in turn emerges from their educational knowledge. Because of each individual`s different developmental level of teaching knowledge and learning knowledge, their experiences, belief, criteria and educational styles must be different form one each other. So, it is inevitable that before participating in educooperational interaction, ‘the more advanced’ and ‘the less advanced’ adjust their teaching knowledge and learning knowledge according to the other`s developmental level of educational knowledge. The modes to adjust may be diverse, including such areas as spontaneous equality, one-way adjustment, and meta-education. This rule can be one of endogenous educational principles. The more we recognize and maintain this principle, the more educooperation will be activated in our life.