Microwave dielectric ceramics are the key basic materials of 5G/6G communication technology, and the materials with a high quality factor (Q×f), a low dielectric constant (εr) and a near-zero temperature coefficient of resonant frequency (τf) become a focus of research and development. In this paper, a series of garnet Ca3-xMgxYb2Ge3O12 (0≤x≤3) ceramics were prepared by a solid-state reaction approach, and the influence of Mg2+ “rattling” effects in the A-site on the microwave dielectric properties was investigated. The ceramics with 0≤x≤2 exhibit a normal garnet structure, εr gradually increases from 10.3 to 11.8, whereas the Q×f gradually decreases from 98 000 GHz to 78 000 GHz, and τf is in the range from -40×10-6 to -56×10-6/℃. The ceramics with 2