'Aḍud al-Dawla (d. 372H/983) was the most powerful ruler of the Buwayhid Dynasty. This paper aims to examine his view of his royal lineage as expressed in Kitāb al-Tājī, which was written by Abū Isḥāq Ibrāhīm al-Ṣābi' (d. 384H/ 994) at the express command of 'Aḍud al-Dawla. It also aims to understand the book within the wider social and political context of the period when Buwayhid Dynasty was at its zenith.<br> Section 1 provides an outline of the content of Kitāb al-Tājī and its background. The book was written when 'Aḍud al-Dawla was just about to send a military expedition in 369H/980 against the Ziyārid and Sāmānid dynasties in the eastern regions of the 'Abbāsid Caliphate.<br> Section 2 establishes the noble lineage of the Daylam and the Jīl people by reconstructing the contents of Kitāb al-Tājī, which is now available only in an excerpt version. I compare the excerpt with some later sources that quote directly from the original text of Kitāb al-Tājī. These sources suggest that not only the Daylam but also the Jīl were connected with the Buwayhid Dynasty through a marriage alliance.<br> Section 3 further analyses how the writing of Kitāb al-Tājī was affected by the ongoing conflict between the Buwayhid and the Ziyārid. When 'Aḍud al-Dawla ordered Abū Isḥāq to write Kitāb al-Tājī, his underlying agenda was to present his lineage in order to justify his reign vis-à-vis the Ziyārid and their allies.<br> In conclusion: (1) Kitāb al-Tājī reflected 'Aḍud al-Dawla's view of his own royal lineage, and, crucially, it was written under his supervision. (2) He portrayed himself as the converging point of the two noble lineages of the Daylam and the Jīl. (3) His purpose was to justify to his own army and to the Daylamite and Jīlite principalities the legitimacy of his reign just before lauching his military expedition against the Ziyārid of the Jīl.