Recently, there has been increasing interest in pushing the operating frequency of RFID systems toward microwave frequencies (5.8 GHz) to make use of the ISM band. This work addresses the design of miniaturized fractal antennas for this application. Different fractal antennas based on aperture-coupled feeding technique are studied, analyzed and compared for size reduction, low profile, and good omnidirctional patterns. Further, a new design methodology for miniaturizing fractal RFID antennas is introduced which uses particle swarm optimization technique, implemented in MATLAB, and CST Microwave Studio which is an electromagnetic simulator based on finite integration technique. The design methodology is applied to different fractal antennas such as Minkowski slot , Koch slot, and Minkowski aperture-coupled antennas. The methodology is also used to design a miniaturized fractal antenna having approximately perfect impedance matching with the tag chip. The design does not need any additional loading or matching network and hence yields relatively lower cost and smaller antenna area compared with conventional tag antenna systems.