In the modern Chinese, the prepositions "對"and ``向" are considered as a pair of synonyms to introduce an object. However, the research on them focuses more on explanation and less on discrimination with many contradictions. In view of this, this thesis does a research from the perspective of grammaticalization and finds that the prepositions "對" and "向"develop in parallel with "introduction of an object" and "introduction of a direction" as mainlines respectively. They have intersections in the process of development. On the basis of this, the author holds that they don`t have common meanings but are most likely to be replaced by each other only when the prepositional phrases and predicates both contain [+ directional] and [+ object] in the sentences.