In a single feed semi batch reactor effects of reaction conditions, such as TEOS and water concentrations, reactant feed flow rate and agitation speed, on the silica supersaturation and particle size synthesized in sol precipitation were investigated. As the TEOS and water concentrations increased, the particle induction time and the supersaturation at the particle induction point were reduced. However, the dependencies of the particle induction time and supersaturation at the particle induction point on the reactant flow rate were opposite to each other. That was, the particle induction time decreased with increase in the reactant flow rate but the supersaturation at the particle induction point increased. The fact that the supersaturation level to induce the particle was varied with the reaction condition implied that the solution condition determined by the reaction condition as well as the supersaturation level was important to the particle induction. Meanwhile, the particle induction time and supersaturation at the particle induction point were little influenced by the agitation speed in the reactor. This result was due to the micromixing time being short enough in comparison to the reaction time of TEOS. Thus, the particle induction time and supersaturation at the particle induction point was predominantly determined by the reaction time rather than the micromixing time of TEOS. The reaction rate constants of TEOS hydroysis and condensation, which was predicted with Kim et al.'s equations[15] at the reaction conditions was described as a function of the particle induction time. In addition, the particle size synthesized at the reaction conditions was able to be well fitted as a function of the particle induction time.