In an atmospheric-pressure dielectric barrier-discharge (AP-DBD) source, we considered the operating condition to achieve the maximum power dissipation and photoresistor (PR) ashing rate. The AP-DBD generated in a two plate electrodes cell was employed to observe the nature of discharge. Since the discharge can be sustained for a long period with a higher reactor capacitance, the dissipated power and PR ashing rate could be maximized at a specific operating frequency by varying the reactor capacitance. Therefore, the operating frequency and the reactor capacitance play an important role in the optimization of the reactor operation and the PR ashing process. The reactor capacitance was determined from the reactor configuration, the electrode size, the gap between the electrodes, and the dielectric properties of the insulator on the electrode. The result will be applicable to the development of a large-scale process reactor using AP-DBD