The clock regenerator and backscatter modulator for Ultra High Frequency (UHF) RFID application are implemented. The whole circuit consists mainly of a current source and some delay cells. Due to variations of power supply vohage under various circumstances, special optimizations of the current source are made by the author so that delay values under different power supplies will not v~0- much. The circuits are realized under 0.18ym process technology. Simulation resuks under different conditions manifest that the design circuits satisfy the system requirement.