In this paper, a high accuracy, high efficiency, and wide current sensing range on chip current sensing technique is presented. The proposed current sensing circuit uses simple switch technique to achieve high accuracy. The current sensing circuit has no OP amplifier so that it would not reduce the power efficiency. The test chip is fabricated in TSMC 0.18μm 1P6M 3.3V CMOS process. The measurement results show that the buck converter with on-chip current sensing circuit can operate from 700KHz to 1.4MHz with supply voltage from 2.5 to 5 V for lithium ion battery application. The accuracy of proposed current sensing circuit is higher than 89.8% for load current from 50 to 500mA and for temperature from 0 to 100°C. The power efficiency of the buck converter is up to 91.2%.