Making use of qualitative data collected from two elementary schools, this study examines main factors that affect the quality of mentoring in those schools positively and negatively. Major findings indicate that instructional mentoring in two elementary schools had taken largely different forms, and thus produced varied effects. Concerning master teacher-related factors that affect instructional mentoring, their experiences and perspectives as mentor teachers influenced mentoring procedures and outcomes. This is because master teachers in two elementary schools had different perspectives on what good teaching is and how to help beginning teachers. On the other hand, beginning teacher-related factors such as their recognition of needs, characteristics of assigned classroom, participating attitudes, and work experience influenced their experience of being involved in mentoring. In addition, principals’ view on the significance of mentoring and the school-wide professional culture also appeared to affect instructional mentoring. Based on these results, this study provides several implications that are useful in implementing mentoring in more fruitful ways.