Economic diplomacy, using relations and influence to improve international trade and investment, covers a wide range of international representatives, domestic institutions, as well as bilateral diplomatic activities. Economic regionalism is the result of dynamic economic diplomatic interactions that can take shape in the form of a regional organization. The Economic Cooperation Organization (ECO) is the only major regional organization to which Iran is a founding member, and sometimes manifests three specific features of the 11th government's foreign policy: developmentalism, regionalism, and internationalism. Now the question arises that what are the factors affecting the inefficiency of Iran's economic diplomacy in the ECO organization in advancing the economic regionalism policy of the 11th government? The main hypothesis is that in spite of the great potential in ECO, intra-structural problems, parallel work and overlapping of the Ministry of Foreign Affairs with other institutions have been the cause of inefficiency and inability of this ministry to advance the policy of economic regionalism of the 11th government. The data of this article have been obtained by documentary-archival method and estimated using Eviews software. The analysis of the results obtained by interviewing Iranian diplomatic elites has been analyzed and explained.