The objective of the research is to explain the implementation of CSG in attempt to develop teacher professional competence. Besides, the research is also designed to examine the effects of CSG implementation on the teacher professional development. The research was carried out at Madrasah Aliyah Negeri (MAN) Karanganyar Central Java, from April to September 2014 employing participatory action research. The subject of the research is seven English teachers of Madrasah Aliyah Negeri Karanganyar. The data were obtained from several techniques including test, participant observation, in-depth interview, questionnaires and document analysis. To analyse the quantitative data, I applied a descriptive statistics and constant comparative method for qualitative data. The findings reveal that the implementation of collaborative study group is effective when the principles of andragogy theory are applied and that CSG enhances teacher professional development.. It implies that collaborative study group is very important in developing teacher professional competence. Therefore, it is recommended that English teachers should be actively involved in collaboraative study group in order to improve their professional development.