Oxides of the type, Sr3Zn1−xMgxNb2O9 (0≤x≤1) have been obtained by the ceramic method. These oxides crystallize in the hexagonal cell corresponding to ordered triple perovskites. Sintered disks show nearly frequency-independent dielectric constant for all the compositions. Compositions sintered at 1425°C yield dielectric constant of 20–22 at 6 GHz, with quality factor ranging from 1300 to 1500. Sr3Zn0.5Mg0.5Nb2O9 shows a very low temperature coefficient of resonant frequency (τf) of +4 ppm/°C.