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In this paper, the aperture-coupled circular-polarized microstrip antenna for RFID reader system is presented. The feed structure is the slot-coupled configuration using single microstrip line, which can be designed and produced easily. This structure can also reduce a size of the antenna. A kind of pinwheel-slot is proposed as a shape of slot on the ground plane. Iterative simulation is performed to get an optimal design for the antenna structure. A few of critical parameters is investigated to find the effects on the input impedance and CP axial ratio. The simulation results for LHCP and RHCP operations are also compared. The same technique can be used to characterize other parameters of the antenna for RFID system.