Four MgO-Ta2O5 ceramics with the MgO/Ta2O5 mole ratio x = 1, 2, 3, and 4 were prepared by traditional solid-state reaction method, and the influence of x on the phase composition, microstructure, and dielectric properties (the dielectric constant epsilon(r), the temperature coefficient of resonant frequency (f) and the quality factor Qf) of the materials was investigated using XRD, SEM, etc. The results indicated that the ceramics were composed of two crystalline phases MgTa2O6 and Mg4Ta2O9 in the composition range studied, and that the dielectric properties ln epsilon, 1/Qf, and (f) changed proportionally to the fraction of main crystal phases, which meet perfectly with the mixing model proposed in this study. It is obvious that the proportion of the two crystal phases could be precisely controlled by x, and thereby, the dielectric properties can be conveniently and precisely tailored. Our research provided a new microwave dielectric ceramic with the composition of 2MgO-Ta2O5, which has an ultrahigh Qf value (211 000 GHz), low dielectric constant epsilon(r) (19.9), and near zero temperature coefficient of resonant frequency (f) (8 ppm/degrees C).