A class of bandwidth-enhanced full-metal cavity-backed slot filtenna is proposed in this communication. The design is constructed by three resonators. They are two resonance windows and one resonance cavity. As a consequent of that, three resonance modes are achieved using a single cavity. By using this resonance window, the 10-dB frequency bandwidth of the proposed cavity antenna has wide fractional bandwidth of 20%. To the authors' best knowledge, the proposed filtenna achieves the widest fractional bandwidth among those published full-metal cavity-backed slot filtennas. Moreover, a fifth-order filtenna is also designed to further broaden the 10-dB fractional bandwidth to 24%. Finally, an antenna prototype was fabricated and tested for experimental verification of the proposed design methodology. Good agreement between measurement and simulation shows the feasibility of the proposed wideband full-metal filtenna using resonance window approach.