In this article, a design concept to take advantage of two coupled slots and their inductive coupling to provide the necessary $90^{\mathrm {\circ }}$ phase difference for circular polarization (CP) realization is presented. Initially, two individual identical slots are arranged in coupling to each other in a perpendicular position, thus acquiring a pair of orthogonal linear polarization (LP) with equal amplitude. Simultaneously, an inherent $90^{\mathrm {\circ }}$ phase difference is naturally produced by the inductive coupling between two slot radiators. In order to clearly illustrate the evolution principle of this proposed CP realization method, the whole structure is in general modeled as an equivalent three-port network for efficient study. Based on these two achieved conditions, a CP slot antenna with one minimum AR point of 4.7% AR bandwidth (ARBW) is satisfactorily realized. After that, based on this proposed design concept, an additional slot is introduced to construct a new inductive coupling, thus facilitating two AR points under triple-mode resonance. With this arrangement, a wideband CP slot antenna with an improved ARBW of 14.1% is successfully implemented, which in turn validates the effectiveness and flexibility of this CP design approach. Finally, both the CP slot antenna element and the wideband CP antenna are implemented, fabricated, and tested. The measured results of the antenna prototypes are found in good agreement with the simulated results, thereby confirming the design concept.