The purpose of this study is to analyze the relationship between geology and geothermal gradient in South Korea using GIS. For the analysis, 352 temperature logging wells were constructed to spatial database and the relationships beween geothermal gradient and geological time and lithology were analyzed using the overlay the wells layer and 1:1,000,000 scale geological map layer. The average of the geothermal aradient of South Korea is 29.34$^{circ}C$/km. In the geologic sequence, Cenozoic strata has 39.7$0^{circ}C$/km, Mesozoic strata has 30.63$^{circ}C$/km , Paleozoic strata has 22.32$^{circ}C$/km, Proterozoic strata 23.15$^{circ}C$/km geothermal gradient value. In the lithological aspect, plutonic rocks 33.96$^{circ}C$/km, sedimentary rocks have 24.78$^{circ}C$/km and sedimentary and volcanic rocks have 26.85$^{circ}C$/km geotermal gradient value. The result can be used to develop geothermal energy and hot spring as a reference.