The microstructure evolution of Al-Cu alloys with different compositions is investigated in directional solidification. Two distinctly different microstructure evolution mechanisms are obtained in Al-0.85%Cu and Al-4%Cu systems, respectively. In Al-0.85%Cu alloy, it shows a peculiar phenomenon, the microstructure evolution is always cells and no dendrites are obtained. In other words, the cells undergo several transitions as the pulling velocity increasing from 15μm/s to 300μm/s, they follow the sequences: mixed microstructure of pox and banded cells →mixed microstructure of polygonal and banded cells →banded cells →elongated cells. Particularly, it is studied what caused the dendrites not appear in Al-0.85%Cu system. Finally, it comes to the conclusion that there are two aspects which lead to the phenomenon: a dilute alloy composition and a relatively higher temperature gradient.