A wideband vertical microstrip-to-micrstrip transition is presented by utilizing the composite microstrip/slotline resonators. With the coupling between two 50-Ω microstrip lines on top and bottom layers, respectively, and a uniform slotline resonator on the common ground plane, a proposed three-pole wideband filtering performance for the signal transmission through different layers can be achieved. To verify the expected results, a vertical transition prototype is simulated, fabricated and measured. Good agreement between the simulation and measurement results is gained. An operating band from 3.51 to 8.25 GHz with maximum insertion loss of 1.5 dB and group delay<0.4 ns is achieved in measurement.