A novel system Li3Mg2(Nb(1-x)Mox)O6+x/2 (0x0.08) microwave dielectric ceramics were fabricated by the solid-state method. The charge compensation of Mo6+ ions substitution for Nb5+ ions was performed by introducing oxygen ions. The X-ray diffraction patterns and Rietveld refinements indicated Li3Mg2(Nb(1-x)Mox)O6+x/2 ceramics with single phase and orthorhombic structure. Micro-structure and density confirmed that the grain of Li3Mg2(Nb(1-x)Mox)O6+x/2 ceramics grew well. In addition, the permittivity of Li3Mg2(Nb(1-x)Mox)O6+x/2 ceramics with the same trend as density decreased slightly with increasing Mo6+ ions content. However, the Q*f and (f) were obviously improved with an appropriate amount of Mo6+ ions. When x0.04, the Q*f was closely related to the bond valence of samples, while when x0.06, the Q*f was closely related to the density of samples. The variations of (f) and oxygen octahedral distortion were the opposite. In conclusions, the Li3Mg2(Nb0.98Mo0.02)O-6.01 ceramic sintered at 1200 degrees C for 6hours exhibited outstanding properties: epsilon(r)similar to 15.18, Q*f similar to 116266GHz, (f)similar to-15.71ppm/C-o.