The origin of the Slater–Pauling curve M t =Z t −28 (Here M t is the total spin moment and Z t is the number of valence electrons) in half-metallic Heusler alloys Mn 2 CuZ (Z=Ge and Sb) has been studied in detail. In Mn 2 CuZ the half-metallic gap has a similar origin like half-Heusler alloys. The Cu atom acts as an electron “donator” in Mn 2 CuZ, which contributes five d-electrons to the minority spin band of Mn 2 CuZ. So there are 14 valence electrons in the minority band of Mn 2 CuZ below the Fermi level. This is the origin of the S–P curve M t =Z t –28. Finally, it is found that, by partial doping of Cu to the vacant site of half-metallic half-Heusler alloys, the magnetic moments of these can be tuned without destroying the half-metallicity. This can be a possible way to design new half-metals.