To illuminate molecular phylogenetic relationships among Eurasian species of the genus Mustela (Mustelidae, Carnivora), we determined nucleotide sequences of the complete mitochondrial cytochrome b gene region (1,140 base pairs). Molecular phylogenetic trees, constructed using the neighbor-joining and the maximum likelihood methods, showed the common topology of species relationships to each other. The American mink M. vison first branched off and was positioned very remotely from the other species of Mustela. Excluding M. vison, the ermine M. erminea first split from the rest of the species. Two small body-sized weasels, the least weasel M. nivalis and the mountain weasel M. altaica, comprised one cluster (named “the small weasel group”). The other species formed another cluster, where the remarkably close relationships among the domestic ferret M. furo, the European polecat M. putorius, and the steppe polecat M. eversmanni were noticed with 87–94% bootstrap values (named “the ferret group”), supporting the history that the ferret was domesticated from M. putorius and/or M. eversmanni. The European mink M. lutreola was the closest to the ferret group. The genetic distance between the Siberian weasel M. sibirica and the Japanese weasel M. itatsi corresponded to differences of interspecific level, while the two species were relatively close to M. lutreola and the ferret group. These results provide invaluable insight for understanding the evolution of Mustela as well as for investigating the hybridization status between native and introduced species for conservation.