This paper is a part in a bigger research project related to ELT quality in secondary schools in Vietnam since the implementation of the new series of English textbooks was officially passed by in 2006, and the innovated direction was paid to task-based language teaching (TBLT). The purpose of this paper is to make argumentation on why, what and how to use mixed approaches method to investigate two subdomains of teacher cognition - teacher perception and teacher belief. Firstly, an introduction in the research project and research questions about secondary school teacher perception of TBLT and teacher belief of textbooks is shortly made. Secondly, the question what the mixed approaches method are like is dealt with. Next, the question why the mixed approaches method should be used in investigating teacher cognition in general and teacher perception as well as teacher belief in particular is answered and illustrated by writer's framework of research paradigm for the suitability and applicability. Lastly, the question how the mixed approaches method should be implemented to answer the research project questions is clarified and argued for its suitability.