The assessment of deformation condition of the Mosha Fault during Middle-Late Miocene carried out using microstructure and thermometry of fluid inclusions within calcite veins in the fault zone. Based on intra-crystalline deformation of quartz grains, calcite twin, and thermometry of primary fluid inclusions in the calcite veins inside the Mosha fault zone, the maximum estimated deformation temperature of the Mosha fault is 350o c during Tertiary. Calculated pressure of the fault zone using fluid inclusion data is an underestimation of real pressure because the calcite veins formed in the last stages of deformation. Considering the tectonic setting of South Central Alborz and assuming a geothermal gradient ranging between 25-35oc/Km for the region from Eocene to the late Miocene, the maximum temperature of 350o c represents the depth between 10 to 14 Km in adiabatic condition. This depth range means a fault zone pressure between 2.5 to 4 kbar for the Mosha Fault zone during Tertiary, which is in correlation to recent seismic data that confirmed maximum fault activity in the depth between 10 to 15 Km.