In this paper we present a variable optical attenuator (VOA) that is based on microbending of a silicon-on-insulator (SOI) rib waveguide. Optical attenuation is achieved by etching away the underlying SiO2 layer in a section of the waveguide and using electrostatic deflection to introduce vertical bending. When a single-mode rib waveguide is bent, the light traveling through it will undergo mode conversion. The amount of energy transferred to lossy modes depends on the curvature of the bending section. This mechanism is studied with the help of beam propagation method (BPM) simulations. In order to achieve a substantial amount of attenuation by bending, voltages in excess of the pull-in threshold are used, bringing a portion of the waveguide into contact with the underlying silicon substrate. An electrostatic zipping action determines the bending radii and the length of the contact region. The equation for the relationship between the bending radius of the waveguide and the controlling voltage is established through the energy method, and is numerically solved. FEM modeling is also performed to validate the result from the energy model. The device is fabricated by conventional silicon processing steps, plus steps to solve the stiction problem. The test setup for the device consists of a home-made interference microscope to monitor the vertical movement of the waveguide under test and align the input fiber to the waveguide, and other instruments to monitor the output from the device and perform the attenuation measurement. The experimental data agree well with both the BPM simulations and the calculations for the zipping actuation. The tested devices show that we can achieve 14dB attenuation over a 1mm span of a bent waveguide.