A diode string as an electrostatic discharge (ESD) protection structure for RF ICs is attractive because of its reduced total parasitic capacitance. This letter reports a comprehensive RF characterization of diodes for RF ESD protection, including S-parameters, parasitic capacitance, and resistance. It is found that a two- or three-diode string may be an optimal RF ESD protection solution due to the balanced overall performance, including ESD protection level, total size, and ESD-induced parasitic effects, etc. An optimized two-diode string for 5 kV ESD protection features a 108.5 fF parasitic capacitance at 2.4 GHz, and is 3680 μm2 in size. The design was implemented in a commercial 0.35-μm BiCMOS technology.