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(Ba 1- x Ln 2 x /3 )(Co 0.7 Mg 0.3 ) 1/3 Nb 2/3 O 3 (0.01 ≤ x ≤ 0.04, Ln=La, Nd) ceramics were synthesized by solid-state reaction process and their sintering behavior, microstructure and microwave dielectric properties were investigated in this paper. The X-ray diffraction results show that all the samples exhibit a single peroskite phase except for the Nd-substituted compositions with x > 0.02. The sinterability and grains distribution are improved by the La-substitution. Though the long range ordering degrees on B-site are similar in the Ln-substituted compositions, the size of ordered domains observed by HRTEM images are quite different. Compared with the Nd-subsituted compositions, the higher Q × f values in the La-substituted compositions are mainly attributed to the better densification and ordered domain structure with lower energy barriers. Both ε r and τ f values increase slightly with the increase of Ln-substitution. A good combination of microwave dielectric properties was obtained in the La-substituted composition with x = 0.01: ε r = 32.5, Q × f = 79812 GHz, τ f = 4 ppm/°C. [ABSTRACT FROM AUTHOR]