Mixed methods approach has developed into the third methodological movement in educational research. Validity in mixed methods research as an important issue, however, has not been examined as extensively as that of quantitative and qualitative research. Additionally, the previous discussions of validity in mixed methods research focus on research design and procedure, rather than validity per se. This paper presents another perspective by using philosophical and methodological insights generated from Habermas' critical theory, especially his Theory of Communicative Action and validity claims. Theoretical assumptions and how they are consistent with principles of mixed methods research are introduced. Three types of validity claims are explained with an example and how they are applied to understanding validity in mixed methods research is further discussed. This paper concludes with implications for educational research and future directions.