The relationship between the microwave dielectric properties and the IR active phonons of xBa(Mg1/3Ta2/3)O3 –(12x)Ba(Mg1/3Nb2/3)O3 ceramics was investigated. The IR modes were assigned, and the origin of dielectric response was determined. Among the 15 prominent IR modes, we found that the normal vibrations of the O layers and that of the Ta/Nb layers are strongly correlated to the measured dispersion parameters, such as the resonant strength ~4pr! and the damping coefficient ~g!. The frequency shifts of the normal modes of the O layers and that of the Ta/Nb layers explain the linear decrease of microwave dielectric constant ~K! as x increases, while the width of these modes correlate with the Q3 f value.