This thesis is a reference grammar of Ersu, a Tibeto-Burman language spoken in the southwest of Sichuan Province, China. Data supporting this work were collected through my immersion fieldwork conducted in Lajigu (腊吉沽), an Ersu village in Bao'an Township, Yuexi County, Liangshan Yi Autonomous Prefecture, Sichuan Province (四川省凉山彝族自治州越西县保安乡). Theoretical framework is mainly based on Basic Linguistic Theory. Ersu is a head-marking, verb- final, tonal and agglutinative language with an isolating tendency. The language has a fairly complex phonology including 37 simple consonants, 22 clusters, seven basic vowels, three rhotic vowels, six diphthongs, one triphthong and two tones. Reduplication, compounding, affixation and cliticization are attested in word formation. Kinship terms, directional nouns, verbs and adjectives may contain a prefix. Suffixes include gender class, causative markers, nominalizers, etc. Ersu has a rich set of enclitics, including evidentials, aspectual markers, genitive markers, agentive markers, locative markers, etc. Nouns, verbs and adjectives constitute open word classes. Classifiers and adverbs form "semi-open" word classes. Closed word classes include pronouns, relator nouns, quantifiers, demonstratives, numerals, modal verbs, negators, onomatopoeias, coordinators, clause linkers and clausal- or sentential- final particles. In an NP, the head noun precedes modifiers such as numerals, classifiers, adjectives but follows genitive phrases and denominal adjectives. Demonstratives may precede or follow a head noun. The nucleus of a VP is a verb that may be followed by a causative, an evidential and an aspectual marker but always follows a verbal action classifier. The canonical constituent order of a simple clause is AOV/SV. However, any one of the constituents can be ellipsed in discourse. The syntactic constituent order may also vary due to pragmatic motivations. Like many other Tibeto-Burman languages in the southwest of China, discourse organization in Ersu is mainly driven by semantic and ...