Abstract: Both the Baseline Theory and the Border Theory can not fully solve the problem of ethical boundary in the communicative action. The Boundary Theory， by jointly establishing the baseline and the border， is likely to provide a solution to this thorny issue. The baseline serves as a basic boundary of ethics field， which is united by the selfconscious rationality of selfdiscipline and the free rationality of heteronomy. The border is an expansible boundary of ethics field， which is united by the limited scope and the limit of width. As a threedimensional boundary of ethics field， it is costructured by personal baselines and social boundaries， and constructed by the subjects inherent limits and the objects external scales.

Key words: baseline, border, ethics field, limit