To clarify the tribocharging characteristics of toner and carrier, the mixing time dependence of the toner charge at various toner concentrations was investigated. A new blow-off tribo-charge measurement apparatus was used to obtain more precise data. As a result, the decay of the toner charge was observed in most cases. A new model is roposed based on the assumption that recombination of the charges between the external additive and carrier caused the decay, and the model was evaluated by numerical analysis. Because the fitting curves derived from this model were in good agreement with the experimental values, it was concluded that this assumption is appropriate.