This paper describes the process and the effect of a beginning mathematics teacher becoming professional through action research. Classroom observations and interviews were conducted and transcribed. The observation notes and students' learning profiles were collected. The way of analysis of the episodes was adopt to analyze the data. We found that this teacher was able to design suitable activities for her pupils making mathematics sense, and tend to teach diagnostically. Her teaching concerns have transferred from task-concerns to pupil-concerns. The development of this teacher's pedagogical power was interpreted by a learning model in which pedagogical reasoning by analogy and pedagogical cognitive conflict interplayed.