This study comprised of teachers with interest in math education, examine their perceptions of colleague coaching experiences regarding process-focused assessment in math among those who experienced coaching by an evaluator and his or her roles in a cyclical manner, and search for plans to increase their instructional abilities for math education and their professionalism in student evaluation. For these purposes, the investigator analyzed the ways that evaluation criteria were reflected at the level of math textbooks based on the achievement criteria in the Revised Math Curriculum of 2015 and carried out process-focused assessment at the level of classroom lessons in “Numbers and Operations” and “Figures” areas in math. The participating teachers engaged in colleague coaching activities twice per week over three months. The research materials were then put to in-depth analysis including their colleague coaching logs, reports on process-focused assessment, observation logs, focus group reports, and individual interviews. The analysis results show that the teachers experienced colleague coaching in the aspect on reflection on evaluation in the process of selecting a method for process-focused assessment, developing a tool for the assessment, and marking and grading the students after process-focused assessment. In the aspect of collaboration on assessment, the teachers discussed a decision-making process to analyze and interpret outcomes in process-focused assessment and the meanings and limitations of the determination process for the achievement level in process-focused assessment. The action research findings demonstrate that colleague coaching activities made a meaningful contribution to the teachers’ improved instructional abilities and professionalism in student evaluation in math education when carrying out process-focused assessment in math under teaching and learning circumstances. In addition, the colleague coaching activities for process-focused evaluation in math provided practical assistance in the development of teaching and learning situations in the aspect of students’ enhanced capabilities in the math subject.