Abstract: Design and phase noise analysis of a MEMS varactor is presented.Metal layer on the substrate is used to control the parasitic capacitance in the configuration.The effect of etch hole on the up-capacitor electrode is simulated using HFSS,and the best size which will not effect the capacitance is found to be 5μm.On the assumption of the constant parasitic capacitance,the phase noise of MEMS varactor is analysed.Based on a SPICE model,simulations of phase noise have been performed using HSPICE.It’s found that the typical phase noise due to Brownian motion is-176dBc/Hz,and acoustic noise is-137dBc/Hz.