In this paper a constant on–time (COT) buck converter with high efficiency and low offset voltage is presented. It was fabricated with a TSMC 0.18μm 1P6M CMOS process. An additional current feedback path (ACFP) technique was incorporated to avoid subharmonic oscillation of the converter without a large equivalent series resistance (ESR) capacitor, thus reducing the output voltage ripple. However, the ACFP causes an offset voltage on the converter’s output voltage dc level with loading changes. The proposed offset cancellation can remove the dc level from current sensor in ACFP. Hence, the offset voltage can be reduced through a proposed offset cancellation technique. The experimental results revealed that the offset voltage of the proposed converter is 8mV under a 0.1A to 1A loading changes. The converter’s maximum power efficiency is 93.4 %, and its output voltage ripple is less than 18mV.