Reduction in Taiwanese A-not-A questions is investigated. In addition to the condition of Deletion Recoverability (DR), the Immediate Dominance Condition (IDC) and the Directionality Constraint (DC) are proposed. IDC specifies that only those constituents immediately dominated by conjuncts can undergo identity deletion. DC specifies that identity deletion cannot operate backward upon A-not-A question except those containing the non-volitional negative morpheme m. In combination with a few additional constraints, DR, IDC, and DC enable us to derive systematically the reduced forms of A-not-A questions in Taiwanese. Some other aspects of the grammar of A-not-A questions in Taiwanese are also discussed.