This paper represents a theory of admittance loaded slow-wave open loop resonators. The main resonator is extended to periodically loaded line. A closed form formulas for the fundamental and the first spurious resonance frequency are derived. Based on the Floquet’s theorem, a dispersion equation of the slow-wave resonator is derived. The resonance characteristics are studied for different impedances of the loading stubs. Dispersion characteristics, defining the filtering function of the periodically loaded line, are also studied for different impedances and electrical length of the loading stubs.