The structural stability. phase compositions and microwave dielectric properties of (1 - x)Ln(1/3)NbO(3)-xLn(2/3)TiO(3) (Ln: La, Nd; x = 0, 0.2, 0.4, 0.5, 0.6, 0.8) have been investigated in this paper by XRD, SEM and network analyzer. Single phase of La0.4667Nb0.6Ti0.4O3 and La(Nd)TiNbO6 compounds with non-perovskite structure were obtained at x = 0.4 and 0.5 composition, respectively The structural stabilities of the perovskites were discussed in terms of the local charge imbalance between the two A-sites. The dielectric constant of La1/3NbO3 is much larger than that of La0.4667Nb0.6Ti0.4O3 or La(Nd)TiNbO6. The Q x f value increased significantly from x = 0 to x = 0.5 and then decreased with the further increase of x. Maximum Q x f values of 49,867 and 25,393 GHz were obtained for LaTiNbO6 and NdTiNbO6 (x = 0.5), respectively The absolute value of tau(f) decreased with increasing x up to x = 0.6 and then increased with the further increase of x. (C) 2009 Elsevier B.V. All rights reserved.