In this paper, a kind of 3D bandpass frequency selective structure (FSS) based on stacked microstrip / slot lines is presented to realize wideband filtering response. By using the stacked microstrip / slot lines, the incident spatial electromagnetic waves can be converted to guided waves in this type of FSS. Then, multi-mode resonators can be constructed under multiple resonances along the guided-wave portion, thus leading to wideband performance with multiple in-band transmission poles. The original design concept is described and the dispersion diagrams of the stacked microstrip and slot lines are given. To verify the design concept, two FSS examples, consisting of stacked microstrip-line and slot-line structures respectively, are designed. Both of them exhibit stable wideband bandpass filtering responses, which have more than 60% fractional bandwidths (|S ₂₁ | ≤ -3dB).