The aim of this study is to examine the impact of the South Eastern Anatolia Project (GAP) on the inter-regional inequality between the GAP region and the rest of Turkey. Cross-sectional analyses are carried out for the years of 1990 and 2000, that is, before the project is put into effect and 10 years after of its implementation. Although this thesis is ultimately concerned with the inter-regional inequality, the within and the total-region inequality are also discussed. Moreover, since not only the economic, infrastructure and service related variables, but also the sociodemographic variables are included in the analyses, it is a comprehensive evaluation, and the results provide current information about the success of the GAP. In this study, Theil&#039 / s inequality index is used as it provides the property of additive decomposability, which allows us to analyze the magnitudes and trends in inequality among regions and within regions as well as total inequality. The indicators include 17 socio-demographic, 12 economic, and 10 infrastructure and service related variables, i.e., total of 39 independent variables. Although the GAP region performed an improvement in absolute terms for most of the variables, the findings show that the inequality between the GAP region and the rest of Turkey increased for more than half of the variables during the last decade. These variables are mostly related to demography (i.e., infant mortality rate, fertility rate, etc.), health services, and GDP p.c.. On the other hand, the between-region inequality decreased for the variables related to infrastructure, urbanization, educational level (i.e., literacy and schooling ratios in primary education), and nonagricultural labor force.