The Hamyerd iron deposit is located in the northeast of Semnan in the boundary of the southern Alborz and Central Iran structural zones. A sub-volcanic body of monzonite and monzodiorite composition intruded limestone and pyroclastic rocks (equivalent to the middle Eocene Karaj formation). The iron mineralization occurred at the contact between intrusive bodies and these sedimentary rocks. The extensive hematite content along with minor amounts of magnetite, goetite, limonite, pyrite, dolomite, barite and calcite are important characteristics of the Hamyerd ore deposit. Mineralization occurred as veins and also hematite lenses with minor magnetite content. Fluid inclusion and stable isotope (S, C and O) studies were integrated to explore the Hamyerd iron ore genesis. Petrographic studies display five types of fluid inclusions in quartz and 4 types in barite. Fluid inclusions in quartz include single-phase liquid, single-phase gas, two-phase liquid-rich, two-phase gas-rich, and three-phase (liquid-solid-gas) inclusions. Three-phase liquid-solid-gas inclusions were not detected in barite. Microtermometry studies in two-phase liquid-rich inclusions revealed homogenization temperatures of 200-250 ˚C and 100-200 ˚C, and salinities of 10-20 and 0.5-5 wt% NaCl equivalent for quartz and barite fluid inclusions, respectively. Microthermometry of halite-bearing three-phase fluid inclusions showed homogenization temperature from 200 to 350 ˚C and salinity from 30 to 40 wt% NaCl equivalent. δ34SCDT values of pyrites at Hamyerd iron deposit are in the range of 2.2 to 7.4‰. The isotopic values of barites range from 13.6‰ to 20.2‰ for δ34SCDT and 10.2‰ to 12.1‰ for δ18OVSMOW, respectively. The carbon and oxygen isotopic values of calcite are in the range of -3.4‰ to -4.5‰ and 17.7‰ to 19.1‰, respectively. Microthermometry of fluid inclusions and stable isotopes (S, O, C) at Hamyerd iron deposit suggested mixing of magmatic and meteoric fluids as origin of hydrothermal solutions. Mineralization in the Hamyerd iron deposit is probably similar to Fe-skarn deposits.