Time-domain modeling of a special kind of passive RFID tag based on surface acoustic waves (SAW), the SAW RFID tag, is proposed. Features of this kind of tags are discussed, as well as its operation principle based on the piezoelectric effect. Preliminary results of the 2D FDTD modeling of a SAW RFID tag, where the reflectors are the electrodes of an interdigital transducer, are presented. The 2D simulation allows accounting the losses introduced by the bulk acoustic waves (BAW). PML boundaries are implemented.