In this article, a novel design approach to wideband dual-mode resonant circularly polarized (CP) antenna is advanced. An analytical design approach with low complexity as supported by a set of closed-form formulas is presented. Thus, the wideband dual-mode CP characteristics can be forward predicted and simply realized by incorporating stub asymmetry-enabled self-phase-shift dipoles and unequal feeding branches. At first, stubs are employed to broaden the impedance bandwidth by simultaneously exciting dual resonant modes. Then, the phase quadrature and equal magnitude conditions for wideband CP can be automatically realized by incorporating unequal Y-shaped microstrip branches and asymmetric slotline stubs. Finally, the coincidence between the simulated and measured results of the fabricated prototype antenna further confirms the feasibility of the design approach. It is shown that the presented antenna can exhibit an impedance bandwidth of 20.1% and a 3 dB axial ratio bandwidth of 16.3% in the boresight direction.