Nano size silica particles were produced by direct oxidation of silicon powder in the water with the base catalysts. The average size of 10~100 nm was successfully prepared from the starting material of silicon powder with variable procession parameters such as silicon solid content, choice of catalyst and amount, the order of mixing, etc. The sodium hydroxide and ammonium hydroxide was chosen for the base catalysts and their amount and addition order were found to control the size of silica particles in the oxidation reaction. The limited amount of NH4OH as the first catalyst seemed to limit the number of nucleus particles, and the limited amount of NaOH further feeded only for particle growing rather than nucleus formation, thereby it was observed to be able to control the average size of particles to be as large as 100 nm. The possible mechanism of particle growth was suggested by solubility of silica and relative affinity of NH4 + and Na+ ions to the surface of SiO2.