Some problems plaguing the current researches on social norms for agent are pointed out in this paper. By importing Coleman' s foundations of social theory, a social norm conception based on rights allocat ion is proposed. The assumption of average rights allocation in Coleman's theory is relaxed, a formal definition for social norms in multi-agent system is give n, and the design of a norm as a constraint satisfaction problem is derived. The n, the rationality of social norms is investigated in this paper, and the condit ions for social norm's constitution and abolition are given. Finally, the ration ality property of social norms under symmetrical interest situation is investiga ted and a value equalization algorithm that can improve the rationality property of a norm is given. The work may contribute to the researches on market-orient ed multi-agent system.