In this study the genitive case and case variations of the subject in a gerund clause are examined to interpret why and how it is thus case marked. Following Abney's (1987) DP Hypothesis that DP and IP are similar in that like I, D is a functional head that takes the maximal projection of a lexical category, i.e. NP in this case, as its complement, we propose that the English gerund clause is actually a DP in structure. With such analysis, the varying case manifestations of the subject in gerund clauses can be accounted for in a theoretical system and the parallelism between gerund clauses and regular complement clauses, finite or nonfinite, can be fully understood.