Abstract: The modality adverb “bu mian” is from verb phrase “to be unavoidable”, which can be further divided into four modality types. “Bu mian1”, used in early Qin period, means “be bound to be”, which is closest to the verb phrase “to be unavoidable” in modality. “Bu mian2”, used in the Wei and Chin period, means “have to”, which is emptier than “Bu mian1” in modality. “Bu mian3”, was used in Tang and Song periods, the verb phrase formed with which is related with psychological activity. “Bu mian4”, used in Song and Yuan periods, emphasizes the natural occurrence of an event with a sense of transition. The occurrence of the four types of “bu mian” one after the other shows the grammatical extension from shallowness to depth.

Key words: modality adverb, bu mian, grammaticalness