In this Paper, word stress in Persian is investigated within the constraint-based framework of Optimality Theory (Price & Smo1ensky, 1993). Earlier studies show that the default stress pattern assigns stress to the rightmost syllable of a word (ke. 'tAb, ge.' reft); however, in the presence of certain affixes, the stress pattern changes (ke. 'tab-am,' na-gereft, na-raft-' an), implying that stress assignment is not a purely phonological phenomenon (Samei, 1991; Ferguson, 1957). I propose that stressable affixes belong to special morphological classes (aspectual, derivational, etc.), that have a corresponding constraint in the grammar which outranks the conflicting phonological constraints, implying that phonology is not blind to morphology. This study is important to cognitive science because it explores the constraint conflicts between different interfaces of the grammar and the interaction between modules.