The synchronous buck converter is generally used as basic circuit for low-voltage application such as POL. The buck converter with peak current mode control has first-order response depending on the power-stage or controller design. Therefore the overshoot and undershoot don't appear in transient response. This feature is the chief advantage of peak current mode control. However, there is no example of the design that employed this feature efficiently. So far, we have performed various discussions about transient response of buck converter with peak current mode control. From various discussions, we have found that the voltage variation at transient time does not depend on the output capacitance on buck converter with peak current mode control. This paper discusses the optimal design of the output filter capacitor for first-order response on buck converter with peak current mode control.