Abstract: The adverb " Yiding” is commonly and widely used in modern Chinese. So figuring out its forming process is of positive significance for us to master its usage from the diachronic perspective. " Yiding” was a loose and non-word phrase before the Tang Dynasty, which expressed the meaning of " once decide/formulate”. The adverb usage that takes on the meaning of " inevitable” budded in the late Tang and Five Dynasties, formed in the Song and Yuan Dynasties, matured after the extensive use in popular literature during the period of Ming and Qing Dynasties, and solidified finally. The lexicalization and grammaticalization process of " Yiding” is a process that its phrase characteristic was constantly weakened and disappeared eventually. In this process, the vocabulary features of the " Yiding” was constantly strengthened and finally solidified.