Expansive soils commonly exist in the arid and semi-arid regions of the world. Climate change causes large volume changes in these soils depending on variation of the soil moisture content. Several design procedures are available for upper structures built on expansive soils in order to prevent the damages due to volume changes of expansive soil. Although there are some methods developed for use in design of slabs resting on expansive soils, project planning information is needed locally in regions where this type of soil exists. The most important parameter of this knowledge is moisture index. For this purpose, many moisture index calculations are given in literature. In this study, Thornthwaite Moisture Index (TMI) described by Thornthwaite (1948) was calculated by using rainfall and temperature records for an average period of sixty-three years of 81 stations of Turkey. The data were received from General Directorate of Meteorology. Later TMI values belonging to each station are arranged in tables. As a result of this comprehensive study, the values of TMI determined for each station were used to prepare a TMI map of Turkey by using Geostatistical Analyst Module of ArcGIS 10.0 software and Kriging interpolation method. The study provides climate of region and the parameters related to structure and soil to be taken into account in design of slabs resting on expansive soils. It is believed that this map would be used intensively for the design of slabs resting on expansive soils in Turkey.