The term or title "Tamghaj" had been used in several resources before the founding of the Ilkhanid government, but researchers have paid little attention to its use and have, thus, failed to explain its significance. However, the repeated use of the name in sources from the first half of the thirteenth century indicates that it had been used to refer to a land which virtually fit that of Mongolia. Yet, since the second half of the thirteenth century, when Mongols and Mongolians were more significantly known and used in dictionaries, the term “Tamghaj” has become obsolete. Nasavi wrote in Sireh Aljalal- aldin that Mongolia is considered as a part of Tamghaj and Jozjani has used the term to refer to the whole of Khita. By using the term "Mountains Tamghaj," Ibn Athir also puts it to good use. Other sources such as History of Jahangosha and Jame-altavarikh, however, have not once used the term. The present paper is an investigation into the application of the term “Tamaghaj” in sources from the thirteenth century.