A series of microwave dielectric ceramics in the compositions of K2Mo2O7, K2Mo3O10, and K2Mo4O13 in K2O–MoO3 binary system with ultra low sintering temperatures were prepared using the solid-state reaction method. Their synthesis, phase composition, compatibility with metal electrodes, microstructures, and microwave dielectric properties were investigated. The K2Mo2O7 ceramic sintered at 460°C with a triclinic structure has a relative permittivity of 7.5, a Q × f value of 22 000 GHz, and a τf value of −63 ppm/°C. The X-ray diffraction patterns indicate that K2Mo2O7 does not react with Ag and Al electrodes at the co-fired temperatures. The K2Mo3O10 ceramic can be sintered well at 520°C with a relative permittivity of 5.6, a Q × f value of 35 830 GHz, and a τf value of −92 ppm/°C. It has compatibility with Ag electrode. The K2Mo4O13 ceramic sintered at 540°C possesses good microwave dielectric properties with a relative permittivity of 6.8, a Q × f value of 39 290 GHz and a τf value of −67 ppm/°C and it is compatible with Al electrode. For K2Mo2O7 and K2Mo4O13, it is found that the grain sizes and the number of grain boundaries play an important role in the dielectric loss. From this study, it can be seen that the three ceramics in K2O–MoO3 system have good microwave dielectric properties, ultra-low sintering temperatures, non-toxic, and low-cost characteristics. So they can be potentially applied to ultra-LTCC devices.