This letter presents a novel three-mode cavity bandpass filter using the corner cuts as a perturbation element. These corner cuts are employed not only to achieve the desired coupling degree between degenerate modes in a single rectangular cavity, but also to create the expected cross-coupling effect for improvement of out-of-band rejection skirt. Based on this triple-mode cavity, a double-cavity filter with five poles is designed. In order to improve the out-of-band rejection and harmonic suppression, a coaxial cavity is further inserted into the two triple-mode cavities to realize a bandpass filter with six in-band transmission poles. Simulated frequency responses are well confirmed by fabricating a prototype cavity bandpass filter and testing its two-port S-parameters.