This paper discusses the problem of the part of speech of zi 自 in Old Chinese and illustrates its grammaticalization path. The part of speech of 自 in Old Chinese has been controversial. This paper argues that 自 is a reflexive adverb, following Pulleyblank (1995), 楊伯峻·何樂 士 (2001), and others. There are two bases for this argument. Firstly, 自 suggests coreferentiality between the agent and the patient, which entails that 自 is a reflexive. Secondly, 自 scarcely appears in subject, object, or adnominal position. Instead, it almost always appears preceding the verb, which shows that 自 is an adverb. From a historical perspective, this paper explains the origin and the grammaticalization path of 自 by analyzing its occurrences in the Oracle-Bone Inscriptions, the Bronze Inscriptions, The Book of Documents, The Book of Odes, Zuo’s Commentary, and The Analects. The original meaning of 自 is ‘nose’(=鼻); it later expanded into ‘self.’ The ‘self’ meaning developed into an emphatic reflexive (meaning an action coming out from oneself) and further developed into a reflexive adverb (meaning ‘selfwisely’). This study shows the development of 自 from an emphatic reflexive into a reflexive adverb by analyzing (i) the sentence structures in which 自 appears and (ii) the verbs co-occurring with 自. While an emphatic reflexive 自 is followed by an intransitive verb or a transitive with or without an object, a reflexive adverb 自 can only be followed by a transitive verb with no object. The reflexive adverb 自 first started to co-occur with verbs of injury such as 殺 ‘to kill,’ 刖 ‘to cut off the foot,’ 刃 ‘to stab,’ 剄 ‘to behead,’ and so on. It is well attested cross-linguistically that words meaning ‘body’ or body parts are frequently grammaticalized into reflexives, in some cases via an intermediate stage of an emphatic reflexive. The grammaticalization of 自 from ‘nose’ (later expanded into ‘self’) to a reflexive adverb via an emphatic reflexive is in concordance with such a tendency.