This paper proposes ``class criticism from a perspective of knowledge``(in short, ``knowledge criticism``), as a new viewpoint and method of class criticism. The knowledge criticism focuses on the revelation of teacher`s knowledge during the class. Therefore, the critic follows up and appreciate the teacher`s behaviour, and then represents the quality and meaning of it. There are several methodological codes in knowledge criticism; connoisseur of connoisseurship, teacher as a main agent, unit and context, experience and reconstruction of knowledge, and selection of representation forms. These methodological codes are useful for perceiving, appreciating and evaluating the revelation of teacher`s practical knowledge during a class. To demonstrate the reality of knowledge criticism, this paper analyzes two class criticism cases, Ahn`s and Suh`s. The knowledge criticism can provide educational practitioners a new method in representing and transacting teachers` professional.