A passive RFID transponder with reflective nonlinear transmission lines (NLTLs) has been developed for capacitive sensor detection. By detecting the phase change in the harmonic signal generated by NLTLs, the capacitive sensor information can be monitored wirelessly. To show the feasibility and design procedure, two open-ended NLTLs with terminating capacitive sensors are implemented in IBM 0.13μm CMOS process. The NLTLs with Bragg cutoff frequency f_B = 10 GHz and 15 GHz achieve an average of 0.25°/fF and 0.61°/fF in the phase of the reflected harmonic signal, when the modeled sensor capacitance changes from 290 fF to 900 fF and from 117 fF to 362 fF, respectively.