This paper describes particle size measurement of silica nano-particles by interactive force apparatus under the electric field in order to suggest the application of the apparatus to the size measurement. The apparatus measures the interactive force between the electrodes filled with suspension of fine particles in order to investigate particle-particle interaction and particle-plate interaction. The apparatus can be also applied for particle size measurement. During the measurement, the electric field is applied between the hemisphere and flat plate, and thus dielectric particles are arranged toward the direction of electric field in the area between two plates. When the two plates are close to each other, two different forces (i.e. repulsive and attractive forces) alternately act on particles due to the change of particle chain structure. The repulsive force occurs when the particle chain structure is stretched by compressive force; whereas the attractive force occurs when the particle chain structure is broken. The cycle of the repulsive and attractive forces is a primary size of particle or size of aggregate, which depends on the degree of agglomeration. In this paper, the degree of dispersion and coagulation of silica nano-particles was evaluated according to the particle size measurement by interactive force apparatus. This apparatus can be used to evaluate the degree of dispersion and coagulation of other nano-particles in various solvents in many process industries.