The electronic devices have been ubiquitous in our daily life. Battery is the power source of all the electronic devices. The battery management system (BMS) is the electronic system to protect the battery and monitor its operation. Current sensing is one of the basic function of the BMS. This function is to monitor the current flowing in and out of the battery. This thesis will focus on the high-side current sensing circuit. The high-side current sensing circuit, by inserting a very small resistor (mΩ) between the positive electrode of the battery and the load, measures the voltage difference across the resistor and amplifies it for the analog to digital converter. This thesis studies the design of amplifier for the high-side current sensing circuit. This kind of amplifier requires low offset, high common mode rejection ratio (CMRR) and power supply rejection ratio (PSRR). Especially, amplifier in the high-side current sensing should be able to suffer the common mode voltage beyond the supply voltage. This thesis presents a design of amplifier for the high-side current sensing circuit. The amplifier operates in the supply range of 3.0V~5.5V. It can suffer a common mode voltage range from 5V~27V. Simulation shows the static current is smaller than 75uA. The input referred offset is smaller the 5uV. It can archive DC CMRR around 140dB and PSRR around 116dB.