A fully integrated system-in-package (SiP) for UHF (860-960MHz) RFID mobile reader is presented. The SiP is made up of an UHF transceiver, a power amplifier (PA), and two baluns. The UHF transceiver was designed in TSMC 0.18µm Si CMOS and the PA was developed with InGaP/GaAs hetero-junction-bipolar transistor (HBT) technology. The transceiver IC is fully integrated for mobile UHF RFID applications and the PA has high efficiency and linearity with 24dBm output power for wireless RFID communications over a distance of 1m. The total size of the SiP module is 11mm × 11mm. The measured data show a transmitter gain of 19.2dB, total current of 253mA and adjacent channel power ratio (ACPR) of -41.8dBc with the second harmonic performance of -57.6dBc. Also, TX-RX leakage was reduced by careful power amplifier and board designs in small SIP module implementation.