The forsterite porcelains were prepared from mixtures of Mg(OH)2 and SiO2 (α-quartz), both of high purity, in the molar ratio of 2:1.2 with and without the admixtures of Kaolin and BaCO3. The mixtures were heated at the velocity of 200°C/hr, kept at various temperatures for 15min., and cooled rapidly in air.X-ray diffraction analysis and the microscopic observation were made for the identification of minerals formed in the fired specimens. For the specimen without the admixtures the formation of forsterite, which was exothermic, was found to set in at about 1100°C and complete at about 1400°C. The formation of protoenstatite began from 1280°C by the secondary reaction of 2MgO⋅SiO2 and SiO2. In the specimen with the addition of 5% Kaolin (PSF-O) which sintered at 1460-1500°C were found forserite, protoenstatite, clinoenstatite, α-quartz, and periclase. In the specimen with the addition of 5% Kaolin and 10% BaCO3 (PSF-B) which sintered at 1300-1420°C were found forsterite, BaO⋅2SiO2, 2BaO⋅3SiO2, and BaO glass. The dielectric losses at 1Mc/s of both specimens, PSF-O and PSF-B, were alike and very small, whereas the value at 10, 000Mc/s of PSF-O was smaller than that of PSF-B.For the practical purpose, the forserite porcelains were also made from sea water magnesia and silicious stone instead of from pure Mg(OH)2 and α-quartz. The base composition was the same as above, but in this case the admixtures were BaCO3, ZnO, and CaO⋅2B2O3 glass. They were added respectively to the base batch by 5 and 10% by weight. The batches without the admixtures were also prepared. The seven kinds of batches thus prepared were calcined respectively until the forsterite forming reaction was completed. They were ground and mixed with 5% Kaolin, and again fired at various temperatures. These fired specimens were examined for their thermal, mechanical, and electrical properties. It was found that the admixtures such as BaCO3, ZnO, and CaO⋅2B2O3 glass had a favorable effect of reducing the dielectric loss of the fired specimen; especially the addition of BaCO3 was most effective. The results of the test of refiring in H2 atmosphere at 1300°C showed, however, that these admixtures in the fired specimens were easy to be reduced and evaporate. Therefore, as the vacum tube the forsterite porcelain without the admixture was recommended.