beta-Ca2SiO4 powders were prepared from commercially available raw materials. CaCO3 and amorphous SiO2 (a-SiO2), by solid-state reactions in water vapor atmosphere. The formation of beta-Ca2SiO4 was accelerated by water vapor. Water vapor is considered to enhance the thermal decomposition of CaCO3 and to attack surface Si-O-Si bonds in a-SiO2 to accelerate the formation of beta-Ca2SiO4. The effect of water vapor atrnosphere on the formation of beta-Ca2SiO4 from a mixture of CaCO3 and a-SiO2) was stronger than that of mechanochemical processing for 30 min.