This paper describes the aspectual markings of Niesu, an understudied Tibeto-Burman language, spoken in Sichuan, Southwest China. Niesu is the dialect of Nuosu, both of which are classified as Nuosu proper. By describing the data of the two subdialects of Niesu, namely Suondi and Adur, this paper categorizes Niesu aspectual markings into two types: simplex and complex. Except for the perfective o⁴⁴ , which is an enclitic, all other aspectual markings are auxiliary. Unlike the simplex markers, the complex aspectual markers are formed through compounding or modification, making them further analyzable regarding the internal structure. Meanwhile, different nuances of meaning or function accompany the complex forms, compared with the basic forms. The grammaticalizations of the aspectual markings in Niesu are cross-linguistically well-attested phenomena, namely the continuous and progressive markings are grammaticalized from the existential verb, ‘be at, stay’, ‘put, place’, the motion verbs and the posture verbs. But the source and formation can be different. For example, Niesu metaphorically makes use of the verbs po⁵⁵ ‘run’ (Suondi) in compound aspectual auxiliary ko³³po⁵⁵nɯ⁴⁴kɯ⁴⁴ (Suondi) to denote the progressive. The functions of the complex aspectual markings are determined by the grammaticalization of the core verbs in the construction.