This paper is based on the systematic investigation of unearthed documents. The character Cao(草) has appeared in the oracle bone script,but all of them were used as Zao(早). Zao(早) appeared in the Qin Series Slips Documents, all of them were not used as Zao(早), but were used as Zao(皂). In the Han Dynasty Wooden slips of Northest Forntier, Zao (皂) was Created by adding a leftfalling stroke on the character Zao(早). The character Zao(早) was used to record the meaning of black. By then, Cao(草), Zao(早) and Zao(皂) played respective roles to record Chinese vocabularies.