Photoelectron spectra of Co2Cn- (n=2, 3) and V2Cn- (n=2-4) were measured in the energy range below 3 eV. Analyses of these spectra by the density-functional theory deduced their electronic states and geometric structures. The growth mechanisms of the 3d transition-metal carbide clusters were discussed on the basis of the structural models obtained. The geometric structures of Co2Cn- exhibit a tendency that carbon atoms aggregate to form a C-n substructure. In contrast, V2Cn-consists of VC2 building blocks, which prelude the formation of a vanadium-carbide network. These features illustrate the differences in the carbide-formation processes of the late and the early 3d transition metals, that is, only the latter forms large metal-carbide networks such as metallocarbohedrens and metal carbide compounds.