子, originally meaning child, is usually a part of a compound or a suffix in modern Chinese. The semantic development of zi varies with its position, e.g. ‘X+ 子’ can be grammaticalized into a suffix, but ‘子+Y’ doesn’t. The issues here are the semantic change of 子 and its mechanisms. We may explore grammaticalization from four perspectives, i.e. syntactic structure, semantics, pragmatics, and cognitive factors, and the semantic one is adopted in the paper. Based on the semantic field theory, both ‘子+Y’ and ‘X+子’ can mean (1) child, (2) person, (3) young animal, (4) seedling, fruit or seed, or (5) something small and hard; but for ‘子+Y’, the meaning ‘subordination’ or ‘affiliation’ is derived, while ‘X+子’functioning as a pure nominal marker appears. As 子develops from a content word to a nominal marker, its change follows the chain (person > object> space>time) proposed by Hein et. al. (1991a,b) and the grammaticalization cycle of Chinese diminutives (Tsao 2003). The mechanisms causing the change of 子include analogy, metaphor, metonymy, etc.