In this paper, a single-feed circularly polarized rectangular microstrip antenna with a slotted ground plane is proposed. To obtain a circularly polarized operation, two ring slots are etched on the ground plane. These slots are optimally placed below the diagonal corner of radiating rectangular patch. Additionally, with the optimum position of the ring slots, either of right-handed CP (RHCP) or left-handed CP (LHCP) operation is worked out. By embedding the two ring slots on the ground plane and generating electric field from the patch, a magnetic resonance can be established in the operating bandwidth of the proposed antenna. Using this phenomenon, two orthogonal nearly degenerate resonant modes for circular polarization (CP) radiation can be excited. In order to demonstrate this phenomenon, surface current distributions on the ground plane for proposed antenna and reference antenna are simulated by using the simulation software SEMCAD X and HFSS [1, 2]. The structure and size of the proposed antenna are the same as those of the reference antenna without ring slots on the ground plane. The proposed antenna shows more improvements on the directivity, gain, and radiation efficiency than the reference antenna. Comparisons between the simulated and measured results are given, and in particular the role of two ring slots is fully explained during the descriptive illustrations.