Abstract: Abstract: Acting as sensors, passive radio frequency identification devices (RFID) tags can avoid sensor nodes bulky and realize battery-powered. So, ultra high frequency (UHF) passive RFID tags embedded in tires have a deep impact on tire life cycle management and tire monitoring，and have been used widely in the USA and the European Union. However, the transmission range is one of the hardest challenges in UHF passive RFID embedded in tires, because the rubber makes it harder to read the tag. The main difference between the common RFID and RFID embedded in tires is the communication medium, which attenuates RF power from the reader in RFID embedded in tires. To improve the transmission range and reliability of RFID embedded in tires, it is of great concern to study power transmission of wave propagation for UHF passive embedded RFID in tires, because the tags do not contain any battery and rely on the electromagnetic field for both power and communication. In this work, we studied the power transmissions for UHF passive embedded RFID in tires. In UHF passive embedded RFID systems in tires, the bidirectional radio link between reader and tags goes through air and tires. The total path loss is caused by several factors: reflection loss due to reflected power at tire-air boundaries, attenuation loss in the tires, and spreading loss which is simply due to the radiation properties of antenna. Each of these factors can be analyzed using the permittivity of tires and tire-air boundary conditions. So, the power transmissions are based on the permittivity of tires and tire-air boundary conditions. We use the OCP (open-ended coaxial probe) method to measure the permittivity of tires. By analyzing the radio link for UHF passive RFID, we establish a model of power transmissions of UHF embedded RFID in tires and make numerical analyses. Numerical analyses show that the error of the OCP methods for measuring the permittivity of tires is small, and the absolute error between the measured reflection coefficients and the simulated reflection coefficients is less than 0.04. It is suggested that the parallel polarization and normal incidence of wave should be chosen for improving the performance of the UHF embedded RFID in tires, and the depth embedded in tires should be chosen reasonably to avoid power transmission function locating valley also. In addition, it is necessary to optimize and design the antenna of tag for the impedance matching of tag antenna and chip. The research can provide a reference for the design of UHF passive RFID tags embedded in tires and TPMS (tire pressure monitoring system).