This work aimed to study the effects of incorporate Nano-metakaolin (NMK) as pozzolonic material on some mechanical properties (compressive strength) of Oil Well Cement (OWC). Nano-metakaolin (NMK) was prepared from Kaolin rock brought by thermal activation of kaolin clay at different temperatures (700–800 oC) for 2 h then crushing and ball milled for (40-60) hours. The cement used in this study comprise of Oil Well Cement class G and NMK were incorporate as a partial replacement additive by NMK of (3%, 6% & 10%) by weight of cement with two different average particle sizes (75nm,100nm) and a w/c of 0.44. Several techniques were used to prepare and characterize NMK Particle Size Analyzers (PSA), X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and the Brunauer-Emmett-Teller (BET). The results showed and approved that the NMK was not only work as filler, but also as an activator to consolidate hydration process, through NMK particles react and consumes calcium hydroxide CH crystals to produce more C–S–H, fills pores to increase the strengths, decrease the size of the crystals at the interface zone and transmutes the calcium hydroxide feeble crystals to the C–S–H crystals, and upgrade the interface zone and cement paste domain.