High-isolation in-band full-duplex (IBFD) cavity-backed slot antennas (CBSAs) in a single resonant cavity are presented in this article. The TE101 and TE011 cavity modes combined with a modified orthogonal feeding structure are first used to design the duplex antenna. The proposed feeding structure can reduce the port-to-port coupling strength and produce a very high isolation. By replacing the crossed radiation slots with two pairs of parallel radiation slots, enhanced radiation gain is achieved. To tackle the narrow bandwidth for the single-mode operation, two orthogonal resonant-iris modes produced by the two feeding slots are introduced to design a dual-mode wideband IBFD antenna. The simulated bandwidth is increased from 0.8% to 5.7%. All the presented antennas with improved performances are designed and realized in a single full-metal resonant cavity. Compared with the conventional design method, high isolation, enhanced gain, and enhanced bandwidth are simultaneously achieved without enlarging the antenna size. Finally, the prototype of the wideband IBFD slot antenna is measured and presented to validate the design concept, which has a 5.8% bandwidth, higher than 95% total efficiency, especially an isolation higher than 72 dB.