Directional solidifications have been done with Al-11.6Cu-0.85Mg (in weight percent) alloy by using a Bridgman apparatus. The maximum, minimum, and average primary dendrite spacing are measured. The results show that the primary dendrite spacing depends on solidification velocity exponentially. The microstructure evolution in the directionally solidified Al-Cu-Mg alloys is simulated by using threedimensional (3-D) cellular automaton (CA) method. The simulation is carried out by fully coupling the kinetic calculations with the thermodynamic calculations. The numerical results are in favorable agreement with the experimental ones. They demonstrate that the formation and growth of tertiary dendrite is determined by the interdendritic solid/liquid (S/L) interface stability as well as the overlap of the concentration fields of the relevant dendrites. The evolution of the dendritic structure during solidification is analyzed in detail. (C) 2014 Elsevier B.V. All rights reserved.