A new microwave dielectric ceramic Ba3La3Ti4NbO18 has been synthesized and characterized. Through Rietveld refinement of X-ray powder diffraction data, Ba3La3Ti4NbO18 is identified as the A(6)B(5)O(18) type cation-deficient perovskites with space group R (3) over barm and lattice constants a = b = 5.6198(1) angstrom, and c = 41.6402(4) angstrom. V= 1138.90(1) angstrom(3) and Z = 3. The crystal structure can be described as consisting of identical perovskite-like blocks, five corner-shafing (Ba, La)O-6 octahedra thick, separated by layers of vacant octahedral. The (Ba, La)-O bond lengths range from 2.526(5) to 3.262(7) angstrom and the (Ti, Nb)-O bond lengths range from 1.784(5) to 2.227(6) angstrom. Further, the effects of ionic polarizability and crystal structure on the microwave dielectric properties of A(6)B(5)O(18) ceramics in the BaO-La2O3-TiO2-Nb2O5 system are discussed. (c) 2005 Elsevier B.V. All rights reserved.