ZA27 alloy solidifies in the way of paste-solidification, and results in gravity segregation. The two-phase flow model was developed on the basis of the solidification characteristic of the ZA 27 alloy. And the governing equations of the solidification process were differentiated and computed in a 3-D cylindrical coordinate system through a heat and fluid simulation software package (PHOENICS). The computation of the concentration field indicated a segregation of Al, and is in good agreement with the experiments. This model could simulate the convection and the gravitational segregation during the solidification.