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The microstructure evolution and mechanical property of Cu-25Ni-25Mn alloy after solution treatment and aging treatment are investigated via TEM observation, XRD analysis and Vickers hardness test. The effect of the NiMn precipitates on hardness and its strengthening mechanism in the Cu-25Ni-25Mn alloy is quantitatively analyzed. The results show that in 450 °C aging process, Ni and Mn precipitate from the copper matrix and form nanoscale NiMn phase particle with a face-centered tetragonal (FCT) structure. The XRD analysis indicates that the NiMn precipitates have a lattice constant of a = b = 0.3693 ± 0.0004 nm, c = 0.3570 ± 0.0006 nm, which is fully coherent with the copper matrix. The precipitation of NiMn phase lead to a precipitation strengthening, which provides significant increase in hardness in the peak-aged Cu-25Ni-25Mn alloy. Compared to the solution treated sample, the hardness of peak-aged sample has been increased by 367 HV. The coherency strengthening and modulus strengthening are the dominant strengthening mechanisms. The hardness increment predicted by coherence strengthening and modulus strengthening mechanism has good consistency with the experiment results.