A new preparation method of particles is introduced, which consists of a semibatch-batch two-stage reaction to decrease the size of TiO2 particles. Using a semibatch process as the first stage, the particle size grows to a certain level (138 nm). However, when using a batch process as the second stage, the particle size decreases about 30 nm. The particles prepared using the two-stage (semibatch-batch) method have a smaller mean particle size and smaller standard deviation than those obtained from the single stage process. It was found that as the concentration of TEOT in the second stage increased, the difference in the particle size between the first and second stages also increased. In this paper, the concentration equilibrium constant (K(eq)) was calculated to predict the effect: of the two-stage mixed system on decreasing the particle size. As a result, the higher the concentration of TEOT in tile second stage, the smaller the K(eq) value. A decrease in K(eq) indicates the occurrence of a reversible reaction. In other words, an increase in the reverse reaction rate constant (k(H,r)) results in a decrease in the particle size. The experimental results (particle size) using the above reaction method could be anticipated with the theoretical approach and produced the smallest particle size (similar to 30 nm in diameter).