— In this paper, a class of 3-D FSS elements, constructed by stacked slotlines loading with strip lines, is proposed to establish systematical synthesis and design process of 3-D FSSs. First, the propagation characteristics of the original stacked slotline are analyzed. Then, by loading with different strip lines, equivalent inductors and/or capacitors are formed with the stacked slotline, which can be used to realize desirable resonators. With the aids of the generalized filter synthesis theory, the proposed FSS elements are designed to achieve lowpass and bandpass filtering responses respectively. The detailed synthesis procedures are given and two synthesized examples are validated by experiments. In final, all the measurements agree well with the theoretical predictions and EM simulations. The proposed 3-D bandpass FSSs feature a few attractive advantages: (i) only single- or double-layer structures needed for arbitrarily highorder filtering response; (ii) equivalent circuit parameters exactly extracted; (iii) stable filtering response under a large variation of incident angles.