Folded quad-mode stub-loaded loop resonators (QMSLLRs) are proposed for realizing both bandpass and dual-band bandpass filters with compact dimensions. The QMSLLR is a folded square loop loaded with four short stubs, providing structure symmetry in both transversal and longitudinal directions. Determined by the lengths of the loaded stubs, the four resonant frequencies as analyzed with even-odd mode method can be either distributed in one passband with equal space, or in two passbands with a guard band in
between, for realizing a single-band bandpass filter or a dual-band bandpass filter, respectively. For both the input and output couplings, two perpendicular feeding lines are parallel coupled to the QMSLLR at one corner. The measure results prove that the structure is suitable for the design of a medium band or even narrow band bandpass filters with compact dimensions.