FDTD simulation of a Surface Acoustic Wave (SAW) RFID tag is presented. The simulated tag consists of a single-electrode Inter-digital Transducer (IDT) with ten electrodes and three single-electrode reflectors, printed on the surface of a 128°YX Lithium Niobate (LiNbO3) piezoelectric substrate. FDTD modeling is done considering the multyphisics nature of the problem, by combining the electromagnetic and the elastodynamics theory in order to obtain the piezoelectric governing equations. Simulation results allow determining the time-position and insertion loss of SAW echoes, which enables the determination of the ID code stored in the tag.