A forsterite powder with superior characteristics which can be sintered at a relatively low temperature can be economically produced, when a magnesium source, a silicon source, and copper particles are mixed to prepare a mixed powder containing 300 to 2,000 ppm by weight of the copper particles, and the mixed powder is fired. The magnesium source used is preferably Mg(OH).sub.2, and the silicon source used is preferably SiO.sub.2. A polycrystalline forsterite powder is preferably produced. The magnesium source, the silicon source, and the copper particles can be mixed in the presence of a solvent to prepare the mixed powder. The forsterite powder preferably contains 300 to 2,000 ppm by weight of copper, has a particle size of 0.20 to 0.40 .mu.m and has a crystal size of 0.034 to 0.040 .mu.m.