Max Horkheimer and Theodor Adorno give a radical account of the enlightenment which appears in Dialektik der Aufklirung. The aim of this paper is to consider the concept ‘Enlightenment’ by camparing it with the concept of ‘Rationality’ which is used in this paper as one of the main concepts in ‘Wissenschaft als Beruf. ’ We will find that ‘Enlightenment’ and ‘Rationality’ have some connections. For example, these concepts have the same meaning in some ways. Further, Horkheimer was influenced by Max Weber. By comparing these concepts, this paper suggests one reading that ‘Enlightenment’ can be interpreted in terms of ‘Rationality. ’