Abstarct: The Kuh-Zar Cu-Au deposit is located in Semnan province and 95 km south of Damghan, and which is geologically as part of Torud-Chah Shirin magmatic belt. The rock types with the Eocene-Oligocene age include of volcanic (andesite, trachyandesite and basalt) and intrusive (granodiorite, quartz diorite and monzodiorite) to gether with quartz veins. The intrusion of felsic to intermediate bodies and hydrothermal fluids into the Eocene rocks caused of alteration and mineralization in this area. Weathering and erosion processes were led to release of gold and other minerals from the source rocks and formed the Kuh-Zar gold-bearing placers. The mineralized type and high potential of the Kuh-Zar area have been the main factors to select the district for studing the effect of size fraction on geochemical stream sediments. In the carrent investigation, heavy mineral, petrography, mineralography and lithogeochemistry studies have been done parallel to stream sediment geochemistry. The Kuh-Zar ore deposit contains a wide range of ore minerals such as pyrite, chalcopyrite, chalcocite, covellite, malachite, azurite, hematite, goethite, and limonite. The vein systems have quartz, quartz-iron oxides and quartz-feldspar compositions. Major and trace elements geochemical patterns are similar in various veins which indicate same origin. The concentrations of Au, Ag, Pb, Cu, Mo, As, Sb, and S in veins are very higher than the upper continental crust baekground, and quartz veins have economic grade of approximately 800 ppb gold. To study of graine size distribution and the relationship with concentration of mextals, 6 sampling stations were selected baxsed on lithology, alteration, mineralization, tectonic and watershed criteries. Sediment samples was sieved into 6 fractions include of 2-0.425 mm, 0.425-0.180 mm, 0.180-0.125 mm, 0.125-0.063 mm and < 0.063mm. Each fraction was analyzed for gold by graphite furnace atomic absorption spectroscopy (FAAS) and for 34 elements by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The study of size fraction shown that 2-0.425 and 0.180-0.125 size fractions were formed the maximum and minimum weight percent of stream sediments, respectively.Geochemical data evaluation demonstrate that the Au ، Ag, As, Cu, Pb, Sb, Mo, S are considerably enriched 700 to 1200 meters away from the source mineralized rocks, respectively.the concentration of these element in 0.063-0.425mm is higher than the other fractions baxsed on the heavy mineral studies, most of the gold grains shape in the Kuh-Zar alluvial deposits are lumpy and films, angular to sub-angular and discoidal to sub-discoidal forms. The gold grain size vary from 100 to 1000 µm, and most of grains are in range of 500 to 1000 µm. the morphology and particle shapes of gold display that these particles have not moved very for from the source rocks. Samples contained a significant number of free gold grains have been collected from about 700 to 1200 meters of gold-bearing quartz veins. Gold is only the economic mineral in the Kuh-Zar placer deposit which accompanied with heavy minerals such as pyrite, Galena, chalcopyrite, chalcocite, realgar, orpiment, wulfenite, mimetite, cinnabar, cerussite, pyromorphite, vanadinite, native copper, malachite, brochantite, jarosite and iron oxides. There is a good correlation between heavy mineral and geochemical inrestingation, and the size of 0.425 to 0.063 mm fractions for was suggested prospecting of gold and associated elements in the Kuh-Zar area.