This paper presents a high-efficiency and fast-transient current-mode DC-DC buck converter using adaptive clock controller (ACC). The proposed technique can dynamically and smoothly adjust the switching frequency during the transient response to improve the undershoot, overshoot voltage, and recovery time. In this work, only an output inductor, capacitor, and feedback resistors are outside the chip. The circuit was fabricated by using 0.35μm TSMC CMOS process. The input voltage is 5V and the output voltage is 3.3V. The transient recovery time and undershoot voltage are less than 6μs and 60mV, respectively, for the load current change from 50mA to 300mA. The maximum conversion efficiency is 94% at 100mA load current. Above 85% conversion efficiency can be achieved for load current from 40mA to 500mA. With the proposed techniques, the performance of the DC-DC converter is improved significantly.