Powders of crystalline α-SiO₂(α-quartz), having the mean particle size of 1-10μm with shapes of rhombohedron, were prepared by hydrothermal synthesis in the temperature range of 250-450℃ for a 3h reaction. The reaction temperature could be reduced as the concentration of a mineralizer increased. Experimental results showed that the particle size became smaller with its narrow distribution as the concentration of a mineralizer increased. However, the particle size became larger with its wide distribution as the concentration of nutrients and the reaction temperature increased. Both of the nucleation and the crystal growth rate also became faster and the particle size became larger as the reaction temperature increased. The above results were expressed as a characteristic equation for the weight mean particle diameter(S) of synthetic powders, such as S≈1.57exp(-E_a/RT)·C_M^-0.33·C_N^0.22·C_S^0.08 with the activation energy, E_a, was 8.15kJ/gmolK.