The attenuation characteristic of a MEMS (Micro-Electro-Mechanical Systems) Variable Optical Attenuator has been studied. Via numerical analysis,the relationship between attenuation and the misalignments (lateral misalignment and longitudinal gap) has been analyzed. It has been demonstrated that the linearity of the device could be improved tremendously via close-loop control system. The key performances of the close-loop controlled Variable Optical Attenuator are measured,including response time (about 1.5 ms),dynamic range (about 35 dB),Wavelength Dependent Loss (< 0.4 dB),and Polarization Dependent Loss (<0.1 dB).