This thesis studied a central figure of Omani Ibdhi thought in the third century AH. It was titled: The Theological Thought of Sheikh Abi Al-Munther Bashir Bin Mohammed bin Mahboub Al-Ruhili and his ethical and cultural Impact. It showed an in-depth analyisis of the most important creedal, ethical and cultural impacts that were left by the Sheikh - may God have mercy on him. This thesis followed the scientific approach in an attempt to uncover the philosophical and intellectual frameworks that hoot Sheikh's views in the issues of constitutional and cultural and political reform. This thesis was divided into three chapters: The first chapter dealt with the Shiekh's theological approach in the matters related to the divinity, prophecies, and the issue of names and provisions. The second chapter discussed the cultural and moral approach by presenting the Sheikh's thought regarding the imamate, the enjoining of virtue and forbidding vice and the method of dealing with the violators. Whereas the last chapter studied the Sheikh's political reformist approach through his biography of the people of Oman in the events of the isolation of Imam Al-Salt bin Malik Al-Kharousi in 272 AH, by highlighting on the reformist thought in its theoretical and practical dimensions from the point of view of Abu Al-Mundhir - may God have mercy on him. This thesis concluded that there is a similarity between what the Sheikh wrote and the ideas of Alkindi, Ibn Sina and Farabi wrote in different philosophical aspects. The research proved that the approach adopted by the Sheikh is close to the modern philosophical methodology pursued by Descartes in some respects. Besides that, In Sheikh's introduction to the concept of Dar al-Kufr we find that he has precisely defined it appropriately to the situation of Muslims at the present time. Indeed, there are a lot of modern and contemporary elements of the political reformist approach that have appeared in what Sheikh wrote on the political side.