Surface segregation is very important for understanding the surface tension of binary systems. In case of a regular solution, the surface segregation is calculated using the Butler model. However, in the case of a complex system, like those forming intermetallic compounds, it is difficult to express the surface segregation. In order to discuss surface segregation in systems forming intermetallic compounds, we measured the density and surface tension of a Si-Cu binary system. We found the effect of clusters in both density and surface tension experimental data. The composition dependence of surface tension was expressed by a modified ideal solution model.