Aiming at the expansive application of RFID technology, design considerations are expatiated to a highly-integrated, multimode, power-adjustable transmitter in mobile UHF RFID reader with CMOS process. The transmitter consists of digital, baseband and RF sections, including an up-conversion mixer and a gain-variable power amplifier (PA). Multiple data modes can be generated in digital section. The direct-conversion RF section is proposed to minimize the off-chip components and provide a low-cost, highly efficient solution. The highlight of the paper is the PA can achieve a maximum output power of 26 dBm and maintain a power-added efficiency (PAE) higher than 40% over the 18---26 dBm power range. Feasibility of the transmitter is validated by ADS simulator.