This is a tentative analysis of the syntactic and semantic change of yiwei. The proto-form of yiwei is proposed to be derived from the adjacent form of yi and ivei, which is semantically comprehended as yi-X-zuo-Y, and contains meanings of both renwei 'regard it as' and dangzuo 'take it as'. The syntactic form of yi-X-wei-Y might result from analogical leveling on the syntactic structure, and hence generates two dichotomies because of the division of labor in meanings and patterns. The first dichotomy occurs when a nominal object corresponds to the meaning of dangzuo 'take it as', and a clausal object corresponds to renwei 'regard it as' , which in turn contains the meaning that speakers report the cognitive content of the cognizer, and the meaning that speakers regard the content as not being correct. The second dichotomy involves on the meaning of renwei 'regard it as'. When being followed by a clausal object in a declarative sentence, yiwei refers to 'wrongly regard it as'. However, when preceding a clausal object in an interrogative sentence, it means 'regard it as'. This analysis can be applied systematically to the change of yiwei in Old Chinese and Modern Chinese. It conforms to the principles of economy and plausibility.