This article presents a basic 3-D frequency-selective surface (FSS) unit cell and its application for synthesis of high-order bandpass filtering response. The proposed FSS unit cell is constructed by stacked slotlines loading with shunted capacitive and/or inductive strip lines, which can be easily fabricated with traditional printed circuit board techniques. First, by analyzing the propagation characteristics of the stacked slotline, it can be found that the stacked slotline is seen as a transmission-line transformer, which can convert spatial electromagnetic waves into guided waves. Then, by loading shunted capacitive and/or inductive strip lines, equivalent capacitors and/or inductors are formed with the slotline section, which can be used to realize desirable resonators for bandpass filtering response. Equivalent circuit models are established and circuit parameters are extracted. With the aids of the generalized filter synthesis theory, the proposed unit cell is first applied to synthesize bandpass FSS with 1/4-wavelength transmission-line inverters. An example of a second-order Chebyshev bandpass FSS with the center frequency of 10 GHz is synthesized and designed. The measured results demonstrate that the desired bandpass filtering response is stable for oblique incidence. Furthermore, to obtain high-order response with a reduced FSS thickness, the proposed unit cell is then applied to synthesize bandpass FSS based on a lumped-element bandpass filter. The detailed synthesis procedure and derivations are given. Another example of a third-order Chebyshev bandpass FSS with the center frequency of 10 GHz is also designed, fabricated, and measured. The measured results show that the desired stable performance is realized and the total FSS thickness is 0.21λ0 (λ0 is the free-space wavelength at the center frequency).