This paper investigates the mechanism for nominative/accusative Case alternations in the siph-ta ‘want-to’ construction in Korean. I argue that the Case alternations in the Korean siph-ta construction are motivated by the peculiar property of siph- that it has dual argument structures and restructuring properties. Specifically, the structural Case on the embedded object is determined by 1) the type of the matrix vP that siph- takes—vP(DO) or vP(BE) - and 2) the presence/absence of the functional category responsible for accusative Case checking, which is selected by the matrix predicate siph-. In so doing, it is demonstrated that the dual argument structure analysis can be extended to account for the same type of Case alternations exhibited by Korean psych-verbs as well as the incompatibility between a nominative object and an embedded psych-verb in the siph-ta construction.