The present study aimed at investigating the impact of conducting a Master's of Education research in ELT on the development of Omani EFL teachers' professional identities (TPI). Besides, it sought to investigate the factors that influence the sustainability of a teacher-asresearcher identity upon the completion of the research and returning back to schools. Hence, data has been collected through semi-structured interviews with six EFL teachers who have completed their M. Ed research and graduated from SQU between 2016 and 2018, their senior teachers or regional supervisors and some university faculty members who have been involved in thesis supervision. Qualitative content analysis has been used for data analysis to search for underlying themes and categories in the data. The findings of the present study confirmed the dynamicity and the continuously changing status of teachers' professional identities. Explicitly, the present study has shown that M. Ed research in ELT had the potential to empower the participant EFL teachers' PIs. TPI Empowerment was manifested in two facets of empowerment: Empowerment of the self and empowerment of others. Empowerment of the self was represented in the development of the identities of 'lifelong learners' who displayed confidence and professional efficacy in their research skills and hence embraced research as a self-directed autonomous form of CPD; and 'research-informed practitioners' whose instructional efficacy has been enhanced inasmuch as their practice has been reformed and informed through their research. Further, empowerment of others was represented in the development of the identity of 'change catalysts' who displayed a commitment to contribute to uplifting ELT policy and practice in Oman at the micro and macro vii levels. Moreover, the current study unveiled three factors that influence the sustainability of a teacher-as-researcher identity; namely, internal and external factors. Internal factors were represented in teachers' self-efficacy beliefs, research-oriented personality and the development of their intrinsic motivation and favorable attitudes towards research as an independent form of CPD. Additionally, the reported external contextual factors were categorized into ministerial and institutional factors which appeared to negatively influence the sustainability of a research identity post the degree. That is, ministerial factors were related to the absence of ministerial follow-up and support and lack of ministerial recognition and incentives while institutional factors were associated with time constraints and workload, the unsupportive school environment and the lack of collaboration between schools and universities in supporting teacher research. For this reason, teachers reported a sense of frustration which could have created tensions and fragmentations in their TPIs. Thereupon, the present study has shown that the negative counterinfluence of the contextual factors has prevailed over the positive influence of the internal factors. Wherefore, in light of the findings of the current study, implications for future practice and future research have been outlined.