Crystal structure and dielectric properties of the A-site-deficient perovskites Nd(1 - x)/3M (x) NbO3 (M = Li, Ag; 0 a parts per thousand currency sign x a parts per thousand currency sign 0.20) ceramics were investigated with X-ray diffractometer (XRD), network analyzer, and impedance analyzer. XRD results showed that no secondary phase was observed for Ag-doped specimens. However, a second phase LiNdNb2O7 appeared for Li-doped ones when x a parts per thousand yen 0.16. The crystal structure at room temperature transformed from orthorhombic (0 a parts per thousand currency sign x < 0.16) into tetragonal symmetry (0.16 a parts per thousand currency sign x a parts per thousand currency sign 0.20) with the increase of x. Complex impedance analysis and the dielectric properties measured at low frequency showed that the dielectric loss in Nd(1 - x)/3M (x) NbO3 (M = Li, Ag; 0 a parts per thousand currency sign x a parts per thousand currency sign 0.20) at low frequency was mainly caused by the lithium or silver ionic conduction, which increased with the increase of temperature. However, the dielectric loss originated from the ionic conductivity was frozen at microwave frequency. The dielectric constant (epsilon (r)) increased with the addition of Ag, whereas it decreased with the increase of Li addition. The pure and Ag-doped specimens exhibited large positive temperature coefficient of resonant frequency (tau (f)), while the tau (f) value changed from positive into negative with the increase of Li addition. The Q x f value increased from 437 to 1,110 GHz in the case of Li doping and 765 GHz in the case of Ag doping, respectively, when x increased from 0 to 0.12. Further increase in x decreased the Q x f values both for the Li- and Ag-doped specimens. Good combined microwave dielectric properties with epsilon (r) = 153, Q x f = 1,110 GHz and tau (f) = -45 ppm/A degrees C could be obtained for the Li-doped specimen when x = 0.12.