This study examined how the National Board Certification (NBC) process, especially the portfolio creation, influenced candidate teachers' pedagogical content knowledge (PCK). In a larger sense, this study aimed to construct a better understanding of how teachers develop PCK and to establish ecological validity of the National Board assessments. Qualitative research methods, most notably case study, were utilized. Participants were three high school science teachers who were going thorough the NBC process. Data sources included classroom observations, interviews, teachers' reflections, and researcher's field notes. Data were analyzed using the constant comparative method and enumerative approach. Findings indicated that the NBC process affected five aspects of the candidate teachers' instructional practices that were closely related to PCK development: (a) reflection on teaching practices, (b) implementation of new and/or innovative teaching strategies, (c) inquiry-oriented instruction, (d) assessments of students' learning, and (e) understanding of students. (Contains 1 figure and 1 table.)