This study aims to evaluate the effectiveness of professional development by H.L Erickson as an educational consultant. For this purpose, this study focuses on history lesson plans presented during a usual teaching day by A.P Magwood̶a social studies teacher. This study attempts to answer the following question: What factors affect lesson improvement of social studies teachers? The findings may be summarized as follows. The aim of the history lesson was “Analyzing U.S. History through concepts to better understand the present.” The teacher refined the construction of the lesson to achieve the aim. In particular, concepts used in the lesson unit changed in quality and quantity. The teacher critically examined and used the theory by the educational consultant. This research has implications for the professional development of social studies teachers.