The crystal structure and dielectric properties of the A-site-deficient perovskites La(1−x)/3AgxNbO3 were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), network analyzer, and impedance analyzer. XRD results showed that no secondary phase was observed in all samples. The crystal structure at room temperature changed from orthorhombic (0≤x≤0.16) via tetragonal (x=0.20) to pseudocubic (x=0.25) symmetry with the increase of x. The dielectric constant (ɛr) and temperature coefficient of resonant frequency (τf) of La(1−x)/3AgxNbO3 ceramic increased with the increase of x due to the decrease of tilting angle of NbO6-octahedron. Whereas the quality factor (Q×f) decreased with increasing silver content as result of the decrease of A-site cation/vacancy ordering. Complex impedance analysis and the dielectric properties measured at low frequency showed that the dielectric loss in La(1−x)/3AgxNbO3 at low frequency was mainly caused by the silver ionic conduction, and that the composition of x=0.13 exhibited largest ionic conductivity and hence highest dielectric loss at low frequency. However, the dielectric loss originated from the ionic conductivity decreased with increasing frequency. Vacancy concentration and ionic conductivity in compounds would affect the order–disorder phase transformation for A-site-deficient perovskites. For the composition with higher ionic conductivity, it would undergo order–disorder phase transformation at much lower temperature. Meanwhile, the ionic conductivity or dielectric loss at low frequency increased with the increase of temperature because of the decrease in cations ordering.