In this article, a new ternary Al-Cu-Sn alloy system has been exploited to fabricate nanoporous Cu6Sn5/Cu composite slices through chemical dealloying in a 20 wt% NaOH solution at an elevated temperature. The microstructure of the sliced nanoporous Cu6Sn5/Cu composite was characterized using x-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and transmission electron microscopy. The experimental results show that multi-phase precursor alloy comprises alpha-Al, Sn, and theta-Al2Cu phases. The new phase Cu6Sn5 emerges through dealloying, and the as-dealloyed samples have three-dimensional (3D) structure composed of large-sized channels (hundreds of nanometers) and small-sized channels (tens of nanometers). Both the large- and small-sized pores are 3D, open and bicontinuous. The synergetic dealloying of alpha-Al and theta-Al2Cu in the three-phase Al-Cu-Sn alloy and fast surface diffusion of Cu atoms and Sn atoms result in the formation of Cu6Sn5/Cu composite with bimodal channel size distributions. In addition, the dealloying duration plays a significant role in the formation of Cu6Sn5 and the length scales of the small-sized ligament/channels at a settled temperature.