The tacit knowledge transition of person-to-person is the radical contents of tacit knowledge transition, knowledge transition, and knowledge management. There is a soft contract constraint on the process of tacit knowledge transition due to the non-coding monopoly of tacit knowledge, and the honesty deficiency of knowledge subject， it leads to a knowledge transition dilemma, and the traditional initiation method is not the best method resolving this dilemma. The reciprocity motivation is gradually becoming the hot spot of economical research and provides the strong challenge to the postulation about “the rational economical men”. There is natural relation between the knowledge transition model and principal-agent model. The optimium transform for the traditional principal-agent model based on the knowledge transition may be realized by planting the reciprocity motivation. The principal may acquire more knowledge transition value, and then improve the knowledge transition productivity of person-to-person.