The electronic structures and stabilities of pure M-12(-) and M-12(2-) were systematically investigated within density functional theory. The nucleus-independent chemical shifts (NICSs) of I-h Sn-12(2-) and Pb-12(2-) are -5.0 and -20.7 ppm, respectively, based on B3LYP/aug-cc-pVDZ-PP predictions, whereas the NICS of Sn-12(2-) is predicted to be 1.1 ppm by B3LYP/LanL2DZ. A startling conclusion is that the NICS4d of Sn-12(2-) and NICS5d of Pb-12(2-) are -5.0 and -7.5 ppm, respectively, suggesting the significant contribution of the inner d orbitals to the total NICS values. This provides the first quantitative evidence for the existence of "d-orbital aromaticity" in Sn- and Pb-based clusters with three-dimensional structures. The d orbitals also contribute to the total NICSs of the K-coordinated clusters. The NICS predictions suggest that larger basis sets including d-orbitals are needed to analyze the aromaticity of some main-group-metal-based clusters (e.g., Sn- and Pb-based clusters) to obtain accurate predictions.