The purpose of this study is to 1) synthetically examine previous studies about teachers' formal learning opportunities, 2) compare the outcome and limitation of empirical studies in regard to this issue, 3) and conduct a meta-analysis to identify patterns among the study results. The results show that first, there has been a growth in the number of studies conducted regarding teachers' formal learning opportunities during last five years and in particular, the themes of teacher training and supervision are the most popular. Second, despite analytical progress including measurement tools for effectiveness, most studies still use limited conceptual frameworks and methods. For instance, most studies define teacher's formal learning opportunities as teacher training or supervision not considering other formats of formal learning. In addition, studies measured formal learning by simply asking participants whether they participated in a specific program or not and the duration of training. Third, the results of the meta-analysis show that both teacher training and supervision are most effective in terms of affective abilities among teachers and the effect on abilities to acquire knowledge and skills are also somehow significant, however, the effect for application of knowledge and skills are not. Based on these analysis results, this study suggests that an active discussion on teachers' learning opportunities, the development of a measuring device of formal learning opportunities, and a design of formal learning opportunities to improve teachers' teaching practice are necessary.