DC microgrid clusters (DCMGCs) that consist of interconnected multiple DC microgrids (DCMGs) point out future evolution of microgrid (MG)-based distributed power systems. Stability is an essential requirement to ensure such complex networks as DCMGCs in safe and reliable operations. However, there are only few works dedicated to stability investigation of DCMGCs. This work proposes an impedance-based small-signal stability analysis for DCMGCs. The small-signal impedance model of the DCMGC is derived, and the stability of the system is investigated through the impedance eigenvalue. The identified unstable conditions by the pole-zero maps are verified by simulation results.