China's ‘minority nationalities’ came into being as a result of the ‘Nationality Identification Work’ carried out in the first decade of the People's Republic of China. The assumption of internal homogeneity of ‘nationality’ inherent in the state project has prevented study of the internal diversity within each nationality. This paper intends, on the basis of field research, to study the internal diversity of the Mongolian society. A preliminary investigation has been made of the ethnogenesis and social structure of the Qotung Mongols in the Alashan region of Inner Mongolia. Through analysis of the relationships between the Qotung and the Mongols and Islam, the author points out that Islam is a major element in the formation of the Qotung Mongols whose ancestors hailed from various groups in East Turkestan. This case study shows that Islam should not be ignored when we study the Mongols and it is imperative that we recognise diversity within the Mongols.