This study investigated the low temperature sintering for microwave magnetic materials [Bi-incorporated calcium vanadium garnets (Bi-CVG), (Bi0.75Ca1.2Y1.05)(V0.6Fe4.4)O-12]. Sintering behavior and magnetic properties of Bi-CVG materials prepared from ultrafine-sized powders were compared with those of materials prepared from micron-sized powders. The activation energy for densification (E-d), derived from temperature and time dependence of materials' density, is markedly reduced by using ultrafine-sized powders as starting materials, i.e. (E-d)(ultra) = 7.04 kJ mole(-1) and (E-d)(micron). = 39.5 kJ mole(-1) for ultrafine and micron powder sintered samples, respectively. The activation energy for densification is further reduced to 4.59 kJ mole(-1) by using microwave sintering process with ultrafine powders. Remnant magnetization (B-r) of the Bi-CVG materials is similar for samples possessing the same high density, regardless of the micro structures. Conversely, the coercive field (H-c) of the materials is larger for samples containing fine grains and vice versa. While the low frequency magnetic properties of the Bi-CVG materials are micro structure-dependent, the microwave magnetic properties of the materials are not. (c) 2007 Elsevier B.V. All rights reserved.