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Radio-frequency identification (RFID) tag using Impulse Radio Ultra Wideband (IR-UWB) is promising for ultra low power consumption and precise localization in indoor cluttered environment. For many logistic applications, ultra low power tag with precision localization is often the most critical requirement. An ultra low-power RFID tag with precise localization is proposed in this paper. The RFID tag is a transmitter comprising of a micro-controller board and a UWB impulse radio board. Power saving and precision localization is achieved by optimization of the circuit design for ultra short pulses as well as system architecture and operation. The tag consumes 20 uA when pulsing at 10 KHz rate with 15.5 dBm peak transmit power. When 1 s sleep mode is incorporated with 1 ms active mode, the average current consumption is 4 uA. The transmitted pulse is captured by energy-detection receivers at the readers. The received signal is sampled in real time. The limited sampling rate reduces the resolution in time delay estimation, and hence deteriorates the ranging precision. To recover the time resolution, curve fitting technique is applied. Measurement in a 6m×6m typical indoor environment demonstrates that the proposed system is able to achieve positioning accuracy of 10 cm. Due to the high sensitivity of the receiver (-71 dBm), the proposed system can achieve a potential reading range of over 100 meters.