This chapter proposes to re-examine the concept of “vernacular” in the May Fourth context. It argues that the Chinese term for the vernacular, baihua, was not a self-evident concept in the May Fourth context, since its meanings were still being contested in the May Fourth period. By tracing the connotations of the term from the late Ming to Qing and early Republican periods, this chapter shows that what we take as baihua nowadays is actually modern invention. This chapter particularly investigates the transformation of the concept of baihua in the May Fourth context by using Hu Shi’s writings as a primary example. The author suggests that concept of baihua in the May Fourth period was not just considered a new instrument or medium of writing, but more as a kind of quality, property, and potentiality that can be used to evaluate or predict the health or life span of any living language.