Professional Development (PD) for teachers is believed to be a critical element to education quality. Conventional PD platforms such as workshops, seminars, and conferences are insufficient to address the needs of teachers in today's rapidly changing world. The Teacher Study Group (TSG) is a promising PD model that may provide teachers with opportunities for active, lifelong learning at the education facility during the workday. This study investigated the effect of engaging six English as Foreign Language (EFL) Omani teachers in the TSG process, and the effect this had on their Reading Comprehension (RC), instructional practices, and on their students' performance. The teachers taught students enrolled in grades eleven and twelve. The study also aimed to document teachers' perceptions regarding their participation in the TSG process and the challenges they faced during the implementation of the program. The six teachers met for approximately 60 minutes at the school site once per week for two months to study a set of research-based teaching strategies in RC. A combination of two designs was utilized to address the current research questions. One group design was used to investigate the effect of this process on the teaching of RC and a quasi-experimental design was used to explore the effect of this process on student performance. A mixed method approach was utilized to answer the research questions. Data were collected through quantitative measures using a pre- and post-classroom observation checklist and pre- and post-reading comprehension tests. The classroom observation checklist was used to measure the frequency with which teachers used effective RC instructional practices. The pre- and post-class reading comprehension tests were used to measure student performance in RC. Moreover, a teacher study group survey and semi-structured interviews were utilized to gather quantitative and qualitative data to explore the teachers' perceptions and the challenges they faced during the implementation of the TSG. The findings of this study revealed that the TSG process had a positive effect on teachers' classroom practice of instructing reading comprehension. Furthermore, the findings showed that the experimental groups whose teachers participated in the TSG process outperformed the control groups whose teachers did not. Regarding their perceptions, teachers reported that their participation in TSG process was a positive and enjoyable experience and time constraint was the only challenge they faced during its implementation. Further research on a larger sample is recommended to establish the effectiveness of this PD model.