Experimental and theoretical studies have been conducted on the effect of the surface coating ratio of surface modified SiO2 particles on the measurement of the contract potential difference between a powder bed and a metal. The applied potential, which is a characteristic property of an electrostatic charge, and surface coating ratio have the same tendency in relation to the mixture ratio of fine to core particles. The surface coating ratio peaks at a certain mixture ratio. On scanning electron microscope photographs, agglomerated particles are observed on core particle surfaces at a high mixture ratio. The surface coating ratio starts to decrease after peaking at a certain mixture ratio, because the fine particles re-entrain. The experimental data agree well with the calculated results that take into account the sticking probability of the fine particles to the core particle surfaces. A theoretical equation is derived for the relationship between the surface coating ratio and the mixture ratio. The relationship between the angle of repose of powder and the mixture ratio indicates the same tendency as the relationship between the applied voltage and the mixture ratio.