In this paper, a frequency reconfigurable antenna using the ON/OFF states of a MEMS (Micro-Electro-Mechanical System) switch, is presented. This structure is based on a multifrequency planar inverted-F antenna (PIFA), designed with an L-shaped open slot etched upon its main plate. As a proof-of-principle, an ON state RF MEMS switch, modeled by a piece of copper of 1×1mm2, is inserted into the slot to achieve frequency switching (OFF state is modeled without this piece of copper). According to the switch position along the slot, the GSM operating frequency is not detuned while other standards become reconfigurable. To validate our concept, a thermal MEMS switch [1,2], designed and developped at CEA-LETI is used. The MEMS switch is presented and its scattering parameters are measured and compared with simulated results from a simple equivalent RLC circuit. The active device is then inserted in the slot to validate the simulated frequency capabilities of the PIFA. © 2005 IEEE.