Rare Earth Elements (REE) consist of the two words "Rare" (means very low concentrations in the earth's crust) and "Earth". The REE are never found as free metals in the earth crust. They are naturally found in minerals that include a combination of various REE metals and non-metals. The REE constitute the largest integrated chemical group in the periodic table and despite of being generally unknown, they are very necessary and suitable elements for a lot of applications.In this thesis, Geological, Geochemical and mineralogical studies were used to study the geochemistry of REE in 1:100000 geological sheet of Kashan prepared by the Geological Survey of Iran. Because of the wide extent of the area, another map with fewer geological units was required. So a new map was prepared by integrating the ages and types of various rock units and according to this new map, 70 samples were taken from various rock units. After collecting rock samples, in order to do petrographic studies, thin and polished sections were prepared and studied using a polarizing microscope. Then, 27 samples were analyzed by ICP-MS method. The analyzed samples consist of Volcanic and plutonic rocks, sedimentary rocks and two skarn rocks. The volcanic rocks include rhyolite, andesite, basaltic trachyandesite, trachyandesite and basaltic andesite. The plutonic rocks include diorite and granodiorite. The sedimentary rocks include limestone, shale, and sandstone. In order to study the lithology of igneous samples, chemical classification charts were used and REE values were normalized to chondrite. The chondrite-normalized patterns of the igneous samples show an enrichment of REE relative to chondrite. Europium negative anomaly in the REE pattern is related to the separation of feldspars. The study of REE show that REE enrichment in igneous rock samples is higher relative to the sedimentary and the skarn samples. The highest REE enrichment is seen in the volcanic rocks. In the next step, to identify the similarities of elements to each other and also to study the geochemical trends between elements, correlation coefficient and cluster analysis of the data were studied. The results show that Zr, P 2 O5, K 2 O and CaO have the highest correlation with REE. Based on this research, it can be concluded that the average amounts of the REE in this study area is almost the same as the average of them in the rest of the world. Also, the light REE enrichment is higher than heavy REE in the studied rocks.