A new dual-mode differential bandpass filter on a squared microstrip ring resonator with a slotline feeding scheme is presented. This filter consists of a microstrip dual-mode ring resonator, four identical slotlines equally spaced along the ring and two differential-mode microstrip line to slotline transitions. By perpendicularly installing the slotline between the differential feeding line and the ring resonator, the coupling strength between them can be dramatically increased as desired. More importantly, only the differential modes can propagate along the slotline, so the proposed filter can naturally block the common modes (CMs) from propagation and achieve good CM suppression. To verify the design concept, a filter has been designed and fabricated. Good agreement between simulated and measured results is attained.