Lim, Sang-bong. 1998. Acquisition of Subject-auxiliary Inversion in Child English and Optimality Theory. Studies in Modern Grammar 14, 349-364. The purpose of this paper is to examine whether the patterns of subject-auxiliary inversion in child English questions can be explained by the constraints in Grimshaw`s(1992) Optimality Theory account of inversion patterns in adult English questions. I briefly review the treatment of subject-auxiliary inversion within the Minimalist Theory. The theory claims that subject-auxiliary inversion is a subcase of head movement that moves an auxiliary across the sentence to the head of the presentential complementizer(CP) position in nonselected CPs. In this paper I try to show that the stages of acquisition in subject-auxiliary inversion and the patterns of inversion in child English. And I also show that an Optimality-Theoretic approach can explain several facts regarding the pattern of auxiliary inversion in child English. In addition, this paper argues that the constraints ranking of child English must be different from those of adult English to capture the characteristics of subject-auxiliary inversion in child English.