Liquid structure of molten pure Cu, Cu-12Al, Cu-12Al-4Ni (mass fraction, %) alloys has been investigated using the X-ray diffraction method. It is found that the main peak of the structure factor of pure Cu is symmetrical. In the front of main peak, the curve takes on a shape of parabola, whereas a distinct pre-peak has been found around a scattering vector magnitude of 18.5 nm?1 in the structure factor of the liquid Cu-12Al alloy. This pre-peak increases its intensity with the addition of Ni in the liquid Cu-12Al-4Ni alloy. The appearance of a pre-peak is a mark of the mediate-range order. Based on Daken-Gurry theory and according to mutual interaction between unlike atoms, the analysis of correlation between different composition and liquid structure was done: the strong interaction exists between Cu and Ni, so Cu-Al can form strong chemical bond which causes compound-forming behavior. Therefore, the medium-range size clusters can form in melt. The presence of the pre-peak corresponds to these clusters. The addition of Ni can strengthen the interaction between unlike atoms and increase the sizes of clusters, thus result in the height of pre-peak increasing.