The purpose of this paper is to investigate the likely effects of the Customs Union between Turkey and the EU on the profitability of Turkish manufacturing industry'. With that aim, the determinants of the price-cost margins of 12 Turkish manufacturing sub-sectors for the years 1994-2000 are estimated by using panel data analysis. Our results suggest that the import ratio has an inverse effect on the price-cost margin of the Turkish manufacturing industry. Similarly, our study also indicates that there is a negative relation between the export ratio and the price-cost margin. Thus, it can be concluded that by creating competitive effect the Customs Union decreased the price-cost margins in the Turkish manufacturing industry.