The substitution behavior of Cr atom in Ni75CrxAl25-x alloy at 873 K was simulated by a microscopic phase-field model. During the precipitation progress, Cr atoms substitute for the Al sublattices partially to form Ni3Al1-xCrx phases (L12 structure). The limits of occupation probabilities of Cr atoms in L12 phases are lower. Cr atom concentration approaches the saturation value in L12 phases when Cr content exceeds 3%. At the ordering domain interface of L12 phases, Cr atoms substitute for the Al sublattices completely, and the stable D022 phases are formed. The final precipitates are mix phases of Ni3Al-Ni3Cr.