Until now, Chinese grammar scholarship has considered 能(不能)VC and V 得(不)C as grammatical structures representing the meaning of possibility and has tried to identify the distinctive features of the two expressions. In fact, although 能(不能)VC and V得(不)C represent 'possibility', this is a secondary function. The main functions represent modalities, which correspond to dynamic modality and epistemic modality, respectively. Based on this point, this paper examined how 能(不能)VC and V得(不)C came to have different modality functions through the transition process from a diachronic perspective, and re-identified the distinctive features between the two structures. In addition, this paper also discussed how different types of modality between the two structures cause differences in discourse contex from synchronic perspective, focusing on utterance focus and practiculity.