Among various trade patterns, intra-industry trade has received special attention from countries due to its specific effects on environmental quality. Given the importance of this issue, this study attempts, first, to calculate the intra-industry trade index for 21 sectors and 98 commodity codes during the period 2000-2023 in the Economic Cooperation Organization (ECO) countries. The data used was obtained from the World Bank database. The study also tries to examine the effect of this index on carbon dioxide emissions in these countries using the panel data method. The results show that the index for Iran has experienced relatively high fluctuations, with a downward trend ranging between 0.10 and 0.0.35. However, the average of this index for Iran has been higher compared to the average of other ECO countries. The model estimation results indicate that intra-industry trade, per capita gross national product, urbanization rate, and energy intensity have a direct and significant impact on carbon dioxide emissions among these countries. Considering the importance of the trade sector, which has always been at the top of every country's strategic economic policies, it is suggested that Iran, in expanding intra-industry trade with ECO countries, review the types of goods it imports and exports as well as its environmental regulations, manage the urbanization rate, and, by using appropriate technology, reduce its energy intensity.