The room-temperature phase composition, microstructural characteristic, and microwave dielectric properties of equivalently substituted LaTiNbO6 ceramics were investigated by means of the Rietveld structural refinement, scanning electron microscopy, and network analyzer. A special interest was focused on the influence of the variation in bond length and octahedral distortion on the phase structural stability. A monoclinic (M) aeschynite phase was obtained in B-site ions (Ta5+ and [W0.5Ti0.5](5+)) substituted LaTiNbO6 ceramics while an orthorhombic (O) aeschynite structure appeared in B-site ions (Zr4+) or A-site ions (Ce3+ and Sm3+) substituted LaTiNbO6 ceramics, which owned typical polygonal and short rod-like grain morphologies, respectively. Compared with the octahedral bond length, a straightforward relation between the phase structural stability and the octahedral distortion was established, in which a reduced octahedral distortion was directly correlated with the destabilization of O or M phases or vice versa, irrespective of the substitution ionic radius. The experimentally observed giant difference in dielectric performance of LaTiNbO6 based ceramics was believed to mainly originate from their distinct phase structure and grain morphology.