In this letter, a strip-loaded slotline resonator (SLSR) is presented and applied to design a class of novel differential mode (DM) bandpass filter (BPF) with improved upper stopband, compact size and intrinsic common-mode (CM) suppression. This DM BPF is composed of three paralleled coupled SLSRs, and two differential tappingcoupled ports. As the loaded-strip length is stretched, the 1(st) resonant frequency gradually falls down so as to make the SLSR compact in size, while the 2(nd) one is tremendously raised towards the 3(rd) one so as to widen the spacing between the 1(st) and 2(nd) resonant frequencies or achieve a wide upper stopband of the proposed DM BPF. To verify the proposed technique, a third-order DM BPF prototype has been designed and fabricated. Both the simulated and measured results have demonstrated good DM upper stopband and CM rejection in addition to compact size.