This paper aims to study a syntactic change of not in the history of English. It is argued that not and its variants were generated in [Spec, NegP] as XP categories in OE and ME, but they began to be reanalyzed as Neg⁰ in the sixteenth century. Noting that the OE and ME negative marker ne in Neg⁰ was missing in some cases, I assume [_Neg e] for Neg⁰ without ne, and propose a licensing condition on [_Neg e]. Then I claim that the condition, which was satisfied in OE and ME, began to be violated in the sixteenth century, and not came to fill in [_Neg e] to avoid the violation.