An experimental analysis is presented to correlate the secondary dendrite arm spacing lambda (2) and dendrite tip radius R with growth rate V and Mg content C (0-Mg) of Al-Cu-Mg ternary alloys. Under constant temperature gradient G (4.84 +/- 0.13 K mm(-1)), a series of directional solidification experiments were performed at five different growth rates V (16.7-83.3 mu m/s) and five different Mg contents C (0-Mg) in Al-5 wt.% Cu-(0.5-5) wt.% Mg alloys. Solid-liquid interface was investigated from the longitudinal sections of the quenched samples, and lambda (2) and R were measured on the dendrite tips. The dependencies of lambda (2) and R on V and C (0-Mg) were determined. The experimental results showed that the values of lambda (2) and R decrease as V and C (0-Mg) increase at a constant G. The present exponent values related to V are found to be slightly lower than the values of the theoretical models and previous experimental works; however, C (0-Mg) exponent values are found to be much lower than the theoretical models and previous experimental works. The ratio of the secondary dendrite arm spacing to the dendrite tip radius is 2.09 +/- 0.15, in good agreement with the scaling law. At a constant C (0-Mg), the values of VR (2) were found to slightly increase with the ascending V. However, as C (0-Mg) increases, the values of VR (2) decrease.