In this paper the author first presents a critical account of some basic views of Habermas' Discourse Philosophy. He points out some difficulties inherent in notions such as valid justification in argumentation theory, in the notion of ideal form of discourses, and in consensus theory of truth. Secondly, he focuses on Habermas' conceptions of validity, acceptance and legitimacy of law from the perspective of neo-institutionalism. In particular, (i) the author argues that Habermas' definition of legal validity is unclear and unrealistic; (ii) the author stresses the distinction between acceptance and acceptability; (iii) Habermas presupposes harmony between sovereignty and human rights postulates, but the present author takes into account the possibility of conflicts between autonomous popular decisions and human rights which must be resolved by methods of discursive democracy; (iv) criteria for acceptance of law cannot be fixed by a stipulative definition, but are in social discussion; (v) legitimacy is not an objective feature of valid law and presupposes an evaluation based on our political convictions.