The rules of origin are justified by the need to regulate trade between two or a group of countries (trade policy mechanism). This paper proposes to study the impact of rules of origin on the implementation of the preferential trade agreement between Morocco and the United States of America. Following this study, the results showed that the rules of origin diminish substantially the efficiency of this Agreement and deprive Moroccan companies of the tax benefits in terms of access to the U.S. market. Therefore, they reflect protectionist practices in term of non tariff barriers. They take the form of manufacturing conditions hard to be met by the operators. The analysis of the relationship between preferential Moroccan exports to the United States of America, on one hand, and the presence of tariff preferences and the Rules of Origin, on the other hand, suggest that if the tariff preferences have indeed the effect of encouraging exports, the Rules of Origin have the reverse effect.