Control of tribocharging characteristics of toners plays an important role in electrophotography. The maximum toner charge may be restricted by the dielectric breakdown of atmosphere gas. In order to clarify this idea, the influences of particle size and atmosphere on the tribocharging characteristics of spherical toners and acrylic powders were examined in this study. The agitation of toner and acrylic powders with copper shots and blow-off measurements were carried out in a shielded box, in which dry air, argon or helium was filled at various pressures. It was found that the tribocharge, Q/M, of the toner and acrylic particles depended on their particle size, the atmosphere gas and its pressure. The tribocharge of toner particles was larger than that of the acrylic particles at the same particle size, which may be attributable to the CCA contained in the toner particles. The tribocharge of toner and acrylic particles increased with an increase in the pressure of atmosphere gas, which suggested that the maximum charge of particles was influenced by the dielectric breakdown of the atmosphere gas. Factors affecting the maximum charge of toner and acrylic particles will be discussed.