An ultra-low dielectric loss ceramics Mg2Ge0.98O4 with olivine structure was fabricated by conventional solid-state route. The phase composition, crystal structure, and microwave dielectric properties were investigated. The phase of Mg2Ge0.98O4 is formed to the orthorhombic forsterite structure with a space group Pmnb (62). The dense microstructure and excellent microwave dielectric properties of Mg2Ge0.98O4 ceramic were obtained at 1360 degrees C for 4 hours, with relative density similar to 96.4%, epsilon(r) similar to 7.3, Q x f = 112 400 GHz, and tau(f) similar to -64.6 ppm/degrees C. The conductive mechanism of Mg2Ge0.98O4 in the low frequency (<1 MHz) was studied by the dielectric spectroscopy and the result with E-dc = 0.93 eV demonstrates that the defect was contributed to the double ionized oxygen vacancies. The intrinsic dielectric properties of Mg2Ge0.98O4 in the microwave region were obtained by infrared reflectivity spectra with epsilon(r) similar to 7.13, Q x f = 120 400 GHz. And, acceptable tau(f) (similar to+2.6 ppm/degrees C) of 0.92Mg(2)Ge(0.98)O(4)-0.08CaTiO(3) composite ceramic was obtained by adding the CaTiO3.