The compositional dependence of microwave dielectric properties has been investigated in the (1 - x)(Na1/2Nd1/2)TiO3-xNd(Mg1/2Ti1/2)O-3 (NNT-NMT) system. The addition of NMT results in significant improvement in the quality factor and the temperature coefficient of frequency, but gradually decreases the dielectric constant from similar to100 for pure NNT to similar to25 for pure NMT. The single perovskite phase is observed with various {hkl} superlattice reflections over the entire compositional range. The increasing tendency of peak splitting with increasing x at some perovskite reflections strongly suggests that the crystal structure of the system changes to lower symmetry structures. This is confirmed using infrared reflectivity spectra. The superlattice reflections related to structural deviation become more predominant as the composition reaches pure NMT. Particularly, {111} superlattice reflections are believed to be associated with the 1:1 cation ordering and responsible for the observed abrupt increase in quality factor at x > 0.7.