The present study concentrates on the cardinal doctrine of Ultimate Reality in the hymns of the Sikh religion’s founder, Guru Nanak. It addresses such important questions as Is the Ultimate Reality as seen by Nanak absolute or personal or both? Is it transcendent or immanent or both? How does Nanak address it or does it have any name? Is this Reality with attributes or without? What is the relationship of this Reality with the visible universe including human beings? and Can this Reality be known? Starting with an introduction, the first chapter makes a detailed study of the key phrase of Mul Mantr, Ik Omkar based on the contents of Nanak’s hymns. Chapter two presents a brief survey of the various names used by Nanak for Ultimate Reality to facilitate proper grasp of deeper meanings of the doctrine. Chapter three examines the various attributes of the doctrine. A detailed glossary has been provided to facilitate understanding of various vernacular terms used in the book.