In this letter, a wideband microstrip-to-microstrip vertical transition using open-circuited slotline stepped-impedance resonator (SIR) is proposed. By controlling the impedance ratio of the slotline SIR, two transmission zeros (TZs) can be properly created and relocated to improve the passband selectivity. To determine the characteristic of this transition, its transmission-line circuit model is presented for analysis and design. Finally, a prototype transition is designed and fabricated for experimental verification. Not only high filtering selectivity via emergence of these TZs but also wideband filtering response with out-of-band suppression of about 22 dB in a frequency range up to f ₀ is experimentally achieved