The oxidation of a quaternary two-phase Cu-40Ni-12Cr-2.5Al alloy formed by the 2.5at% Al addition to Cu-Ni-Cr alloy were investigated at 700~800 oC in 0.1 MPa pure oxygen, and the effect of adding the forth component Al on the oxidation of Cu-Ni-Cr alloy was also discussed. Results show that kinetic curves of the present alloy deviate considerably from the parabolic rate law and the mass gain at 700 oC is higher than that at 800 oC. Moreover, the mass gain of the Cu-40Ni-12Cr-2.5Al alloy is lower than that of the Cu-40Ni-20Cr alloy evidently. The Cu-40Ni-12Cr-2.5Al alloy form uncontinuous external CuO scales followed by the mixed scales composed of Cu, Ni and Cr oxides. Finally, the alloys form a very thin and continuous Cr2O3 scales in contact with the alloy. Obviously, adding the forth component Al to Cu-Ni-Cr alloy is able to decrease the critical content to form Cr oxide and facilitates the formation of continuous Cr2O3 scales under much lower Cr content in spite of the simultaneous presence of two phase in the alloy.