Interface morphology and solute partition during directional solidification process of Al-1.5Cu-3.0Zn alloy were investigated at temperature gradient of 80K/cm and growth rate between 0.1 and 7.1µm/s. The solid-liquid interface was quenched during directional solidification and the micromorphology at the longitudinal section was examed by optic microscope and SEM. The cellular growth interface and dendritic growth interface were observed and characterized. The distribution of Cu and Zn was measured by EDS, the equilibrium solute partition coefficients for Cu and Zn in Al-1.5Cu-3.0Zn alloy were obtained to be 0.31 and 0.58. The activity model and concentration model, developed by present authors, were used to calculate the equilibrium solute partition coefficients in Al-1.5Cu-3Zn alloy. The model-calculated results were compared with experimental data.