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In this paper, the viscoelastic behavior of a tunable RF-MEMS resonator and its impacts are studied by means of direct RF measurements for the first time. This tunable resonator consists of one lambda/2 coplanar waveguide (CPW) Resonator and two nanocrystalline-Ni RF-MEMS varactors. S-parameters of this tunable resonator have been measured for 80 hours under a bi-state bias condition of 0 and 40 V. It is demonstrated that the resonant frequency is shifted by 90 MHz and the varactor deformed by 0.12 1m over the 80 hour period. The gap of the loaded varactor is extracted from the measured S-parameters using finite-element analysis (FEA) tools. A generalized Voigt-Kelvin model is employed to verify the viscoelastic behavior of the resonator. The creep compliance extracted from the RF measurements is in excellent agreement with results in literature.