The one of the secondary minerals of zinc is known namely smithsonite which is naturally occurring zinc carbonate (ZnCO3). The formation of this mineral is controlled by the partial pressure of CO2 .Smithsonite is only stable at elevated CO2 partial pressures [1,2]. Mechanical activation which increases the free energy of the particulate material system by increasing the defect concentration and subsequently the reactivity of the solids has been used in mineral processing. With mechanical activation, it is possible to reduce minerals reaction or their decomposition temperatures [3,4]. Smithsonite from Zamantı – Kayseri, Turkey was subjected to high-energy ball milling. Xray diffraction analysis (XRD) and thermal analysis (TG/DTA) were performed to study the decomposition reactions in both non-activated and activated smithsonite. The results showed that decomposition temperature of smithsonite decreased with increasing mechanical activation time this reaction was completed during 60 min of mechanical activation.