Inrra Industry Trade (ITT) produces extra gains from international trade, over and above those from comparative advantages, because IIT allows countries to benefit from larger markets and to exploit product diversification. In this study, firstly the effects of the post-1980 policies, especially trade liberalization policies on IIT index are analysed by using Adjusted Grubel-Lloyd index and Standard Industrial Trade Classification (SITC) at the 2-digit level. The answer of "whether the customs union membership have promoted IIT index of Turkey or not" question is searched. Secondly, the country specific hypotheses are tested for Turkey by using pooled cross-section and time series regression analysis for Turkey's trade with 14 countries (taking place in the list of top 50 countries of Turkey's foreign trade)