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Structural transformation is a unique characteristic of atomic clusters, but it turns out very different from cluster to cluster. This theoretical study proves that the isomeric transformation between hexagonal prism and hexagonal antiprism is found for the doubly doped Cr 2 Ge 12 cluster but not for singly doped CrGe 12 cluster. We confirm that the ground state of CrGe 12 is the distorted hexagonal prism C 2 h at the 3 B g triplet state instead of various shapes predicted in the previous studies. Upon comparison between the estimation at the B3P86/6-311+G(d) level of theory and the detachment energies measured by photoelectron spectroscopy, hexagonal antiprismatic shape is identified as the most stable isomer of the Cr 2 Ge 12 cluster and it is easy to transform to the hexagonal prism-a less stable isomer by the rotation of the hexagonal rings. That is the first evidence for the structural transformation between a hexagonal prism and an antiprism of the germanium clusters, referring to the ability of Ge-based clusters in the formation of tubular geometry by doping Cr atoms. All the low-energy isomers of both Cr-doped germanium clusters have high magnetic moments. Interestingly, there is a tuning in magnetic properties of Cr 2 Ge 12 from the ferromagnetism of the lowest-lying hexagonal antiprism to the ferrimagnetism of the higher-energy hexagonal prism. The stronger Cr-Cr bond and stronger interaction between the Cr 2 moiety and the antiprism cage are accounted for by the higher stability of the hexagonal antiprismatic isomer.