The purpose of the study was to highlight the role of British Agency in Muscat in the History of Oman in the period (1899-1947 AD) by demonstrating its effect on the various (political, economic, military, and social) aspects of life. The study focused on monitoring the situation of British Agency in Muscat towards competition of local, regional, and international powers. The researcher adopted the historical, descriptive methodology on discussion of the Omani-British relations and narration of historical events. It also adopted the descriptive methodology to study the documents and texts, and to compare the various historical resources to draw conclusions on the subject of the study, and the quantitative methodology on study, analysis and utilization of statistic data in the sides which are needed by the study. The study was divided into introduction, five chapters and conclusion. Chapter was introduction to the study. It addressed the beginnings of Omani British Relations before consolidation of these relations by 1800 AD Agreement which stipulated establishment of British Agency in Muscat and appointment of British Agent in it. Chapter two defined the administrative role of British political agency in Muscat by clarifying the administrative duties of political agents. It examined forms of reports of British Agency in Muscat. Chapter three addressed the political and military role of British Agency in Muscat. Chapter four was dedicated to the economic and social role of British Agency in Muscat. Chapter five discussed situation of the British Agency in Muscat towards competition of local, regional and international powers. The study drew set of conclusions, mainly: British Government endeavored to develop its administrative agency in the Sultanate to deeply establish its presence in the Government of Muscat and to minimize the authority of its rulers and paid all attention to its agency in Muscat. In addition, British Political Agent in Muscat was granted all powers to manage his agency, which enabled him to serve the British Policy in the region in various (political, military, economic and social) aspects, particularly after adduction to Sultan, influence on him and identification of his attitude.