Abstract: In this paper, open-loop resonators loaded by shunt uniform open stubs are proposed to design compact bandstop filters with improved out-of-band rejection characteristics. ompared to open-loop resonators without open stubs, the stub-loaded resonator of the same size possesses lower resonant frequency. Then we analyze the stub parameters affects to the second harmonic suppression of the bandstop filter. A novel resonator with a tunable center frequency and better suppression to harmonic wave can be constructed with lumped variable capacitors between two stubs. In the end, a tunable bandstop filters based on the proposed resonator is designed and fabricated. The center frequency is tunable from 900MHz-2. 65GHz with a lowest parasitic stopband restrained to 8GHz.