In general, the vitality and social functions of a language are assessed in connection with such indices as the language's intergenerational transmission, the absolute number of speakers and proportion of speakers in the population, its present domains of use, its development within diverse domains of use, and the availability of its educational and written materials. The various indices are then ranked for vitality levels and synthesized to obtain the language's degree of vitality and degree of endangerment (De Graaf 2006). By means of a questionnaire survey of Yi-Han bilingual students at the Ethnic Secondary School of Xichang and observations of natural language use by urban Yi-Han bilinguals and rural Yi monolinguals, this study sought to explore current use of Yi in the domains of the family, education, religion, work, and the media in order to assess Yi's vitality and social function in the Liangshan Yi Autonomous Prefecture and proposes measures for enhancing Yi's vitality and social function. (Contains 1 table and 1 note.)