Abstarct: Arsenic (As) is the third most toxic and dangerous substance in earth crust that recognized as a serious threat for human healthy. In this thesis, we investigated arsenic pollution in The Kouhsorkh area located 56 km north of Kashmar and 95 km south of Nishaboor, Khorasan province, NE Iran by used of Geochemical, Mineralogical and Medical geology methods. The main rock formations in the area consist of Tertiary volcanic and various sedimentary rocks. It seems that occurrence of As- Sb- Au- Hg mineralization is one of the most important factors for arsenic contamination in this area. Realizing adsorbent minerals of arsenic and mechanism of arsenic release to water, sediment samples were collected from polluted water sources at the study area. Mineralogical investigation showed the most important minerals in the sediments are Quartz, clay, carbonates, Iron oxyhydroxides (Magnetite, Titano Magnetite, Hematite, Goethite), sulphide minerals (pyrite, Arsenopyrite) and oxidation sulphide minerals. baxsed on the results of ICP-MS analyses the concentration of arsenic was determined between 8.2 to 268.2 ppm in the sediments. Contaminations of As, Au, Cu, Ni, Co and Hg in this area were detected by means of calculated of Contamination Factor. Also Degree of Contamination was defined moderate to high in total samples. According to geochemical conditions such as pH and redox conditions predicted that Alkali-Desorption (AD) is the most important mechanism of arsenic release in this area. Iron oxyhydroxides, sulphide minerals, carbonates, gypsum and clay minerals were found to be the most important sorbents for Arsenic. High arsenic concentration affected health of people in Chalpoo and Avandar villages in Kohsourkh area. Medical geology study showed people in this residential area suffer from skin diseases including: hyperpigmentation, hypopigmentation, keratosis on head, hands and feet. The 24 hour urine specimens were tested for arsenic, the level of total arsenic in urine was 13.66-75.92 µg/L per day that 60 percent of them are higher than permissible limits from 5 to 40 µg/day. More systematic studies are needed to removal of arsenic from water and sediments and cure arsenicosis patients.