Abstract New microwave dielectric ceramics, i.e. Mg 4(Nb 2− x Ta x )O 9 (MNT) solid solutions, were synthesized and their microwave dielectric properties and crystal structure were investigated in this study. From the discrete variational Xα (DV-Xα) method, it was found that the Ta–O bonds in the TaO 6 octahedron become more covalent than Nb–O bonds in the NbO 6 octahedron; this result leads to the decrease of the ionicity in the Ta 5+ ion. The dielectric constants of MNT were slightly decreased from 12.4 to 11.5; this result might be due to the covalent interaction of Ta–O bonding. The quality factors of the samples were found to exhibit high value ( Q· f≒350 000 GHz for x=2) which is comparable to those of Al 2O 3.