The injection pulse width dependence of transient response was studied to investigate the interface states in the Bi- and Pr-type ZnO varistors in more detail using isothermal capacitance transient spectroscopy (ICTS). Although the interface states have been considered to be distributed monoenergetically or discretely, two or three different interface states of the emission process were confirmed by varying the injection pulse width at each measurement temperature. For both types of ZnO varistors, the ICTS spectrum overlapping of the interface states consists of two transient responses: the previously reported trap (Trap 1) and Trap 0 which has a faster emission process than Trap 1. For only Bi-type ZnO, Trap 2 which has a slower emission process, was successfully detected at the longer region of the injection pulse widths tW from 1 s to 100 s. This result suggests that the formation of Trap 2 can be attributed to the existence of Bi2O3. In the case of the application of ICTS for the interface states in ZnO varistors, it is necessary to select the optimal injection pulse width which takes into account the emission processes at each measurement temperature.