Pure magnesium powders together with 0.5 g or 0.75 g of stearic acid were mechanically milled (MM) using a planetary type of ball mill with different processing times. MMed powders were consolidated into bulk materials by the spark plasma sintering (SPS). Changes in hardness and solid-state reactions of the SPS materials before and after isochronal and isothermal heating at 473 K, 523 K and 573 K for up to 8 h have been examined by hardness measurements and an X-ray diffraction. The effects of the amount of stearic acid on the amount of MgO and age-hardening response in the SPS materials were investigated. Enhanced hardness during heating was observed in the SPS materials fabricated from more than MMed 16 h powder, but not in those from less than MMed 8 h powders. In addition, hardness in the SPS materials during heating driven from the amount of MgO formed by solid-state reaction, and the amount of MgO can be controlled by the amount of stearic acid. Furthermore, when the heating temperature increased, solid-state reaction in the SPS materials could be stimulated, subsequently the amount of MgO increased. However, enhanced hardness might be obtained. The thermal stability of pure magnesium produced by MM-SPS process can be controlled and improved by the amount of stearic acid and MM processing time.