Linear dielectrics are widely used to create high power capacitors, where it is a big challenge to achieve high energy storage density in such dielectrics. Here, Ba-based complex perovskite ceramics with high dielectric strength, medium dielectric constant, and ultra-low dielectric loss are proposed as the candidates for high energy storage density dielectric materials, and the significant effects of 1:2B-site ordering and ordering domain structure are systematically investigated. In Ba(Mg1/3Nb2/3)O(3)ceramics, high dielectric strength of 1452 kV cm(-1)combined with high energy storage density of 3.31 J cm(-3)are achieved in the samples after post-densification annealing, and they are 28% and 57%, respectively, higher than those in the as-sintered samples. The significant enhancement of energy storage performance could be attributed to the increased B-site ordering degree, and the uniform ordering domain structure. Furthermore, amorphous alumina thin films are introduced as the charge blocking layers, which significantly enhance the energy storage density to 5.09 J cm(-3). The present work provides a new approach to develop the dielectric ceramics with high energy storage density.