ESD (electrostatic discharge) protection design for RF ICs is a challenging design problem. This paper reports a comprehensive RF characterization of various RF ESD protection structures, including S-parameters, parasitic capacitance and resistance. It is found that a dual-direction SCR type ESD protection structure is the best RF ESD protection solution and an optimized two/three-diode string is an attractive solution as well. A new optimization parameter, F-factor, is introduced to evaluate the overall performance of RF ESD protection structures. A dual-SCR structure of 257 μm2 for 2 kV ESD protection features 43.2 fF parasitic capacitance at 2.4 GHz and F=180. This work is conducted using a commercial 0.35 μm BiCMOS technology.