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The preparation of nanocrystalline aluminum (NC Al) was conducted in two steps. After the NC Al powder was synthesized by an Inert gas condensation (IGC) method in a helium atmosphere of 500 Pa, the NC Al powder was in-situ compacted into a pellet with a 10 mm diameter and 250 μm-300 μm thickness in a high vacuum (10-6 Pa-10-7 Pa) at room temperature. The NC Al samples were not exposed to air during the entire process. After the pressure reached 6 GPa, the relative density could reach 99.83%. The results showed that the grain size decreased with the increased of in-situ forming pressure. The NC Al samples present obvious ductile fracture, and the tensile properties were greatly changed with the increase of forming pressure.