Al-rich Al-Cu-Mg alloys have wide range of applications for automotive and aerospace where the control of the original solidification microstructure is important to achieve desired properties. Under constant temperature gradient G (4.84±0.13 K.mm-1), a series of directional solidification experiments were performed at 6 different growth rates V (16.7 – 166.7 µm/s) and 6 different Mg contents C0-Mg (0 – 5 wt.% Mg) of Al – 5wt.% Cu – (0-5)wt.% Mg alloys. The solid-liquid interface was investigated in as-quenched specimens. Primary dendrite arm spacing λ1 was measured on longitudinal and transverse sections of the specimens. The variations of λ1 with respect to V and C0-Mg were determined and the results were compared to the related theoretical models and the published data.