It is teachers' entitlement as well as responsibility to proceed with lifelong learning and professional development. In Taiwan, the Ministry of Education in Taiwan enacted the regulation for K-12 teachers involving in in-service education in 1996, and has since actively advocated the institutes of higher education to deliver continuing education and offer master's degree programs for in-service teachers; as a result, the number of K-12 teachers studying a master's degree program has steadily increased through the years. This research was developed to explore the purposes of in-service teachers studying a master degree program, the linkage between studying a master degree program and teachers' professional development, and the difficulties confronted by in-service teachers when studying a master degree program. In order to substantively explore in-service teachers' experiences of studying a master degree program and their reflections on professional development, qualitative cases-study was conducted with two elementary school teachers after obtaining their consent; the research techniques used for data collection included teachers’ learning reflective diaries, audio-recorded semi-structured interviews with teachers, and the documents of master's program planning and content. It was found: 1. The teachers studying a master degree program were motivated by salary increase, position ranking, personal honor, and professional development; 2. The teachers studying a master degree program were given the opportunities to apply the theory into practice and enhance their teaching practice; 3. The teachers studying a master degree program were often confronted by lacking a specific target and self-reflection on their professional development.