Self-interference has been a serious problem in passive RFID systems. We propose a new solution using nonlinear transmission lines (NLTLs) for efficient second and third harmonic generation. Three designs for different frequency bands, in discrete and distributed lines, are implemented in IBM 8RF 0.13µm CMOS process. The NLTL with Bragg cutoff frequency of 10 GHz shows −10 dBm second harmonic and −15 dBm third harmonic power by 0 dBm small signal input with a layout area of 0.4 mm by 1.8 mm. This demonstrates the advantages of compact size and passive operation of NLTL in RFID transponder applications.