A novel wideband bandpass ¯lter on a triple-mode slotline ring resonator is proposed in this letter. By attaching two stubs with di®erent lengths and/or widths to the symmetric plane of a slotline ring resonator, the frequencies of ¯rst three resonant modes can be rearranged towards quasi-equal separation. By feeding this slotline resonator using the microstrip feed lines at positions with an angle of 90± to the symmetrical plane, these three resonant modes can be simultaneously raised up, aiming to form up a wide passband. Meanwhile, a wide upper-stopband can be realized by setting the lengths of two stubs unequally. After the principle of an initial wideband ¯lter is described, a prototype of compact ¯lter with internally-loaded stubs is designed with fractional bandwidth of 66% at center frequency of 3.0 GHz. Measured results well validate the predicted ones.