Microstructures of Ba(Mg1/3Ta2/3)O-3 (BMT) materials were examined using high resolution and conventional transmission electron microscopy (TEM) as well as electron diffraction and microstructural characteristics were correlated with microwave dielectric properties of materials. TEM investigations exhibit different types of defects observed at large grains in over-sintered specimens, including dislocations, planar defects and three-dimensional defects. Trace analysis and displacement vector determination imply that stacking faults lying on {1 1 1} planes with a displacement vector of [1 1 0] type may form in the materials. The stacking fault is probably induced by diffusion discrepancy during ordering development of BMT materials which exhibit a concentration wave composition fluctuation at the initial stage. The defects may account for the degradation of over-fired BMT materials. (C) 2002 Elsevier Science B.V. All rights reserved.