The tribocharging behavior of toners in the toner/carrier system dictates the final output results of the electrophotographic system. However, the available theoretical models describe only the charging results in the equilibrium state and leave the charging during the transient state almost untouched. Therefore, in this paper the charging behavior of a toner-carrier development system were carefully examined and a dynamic model that could account for the charging process of both the transient and equilibrium states was constructed. The influences on charging due to the physical parameters of charging time, toner sizes, and toner concentration were studied by measuring the charge-to-mass ratio (Q_t/M_t). The relationship between charges and charging surface was also studied. The predicted dynamic results from the model are in good agreement with the observed data.