A novel variable optical attenuator is proposed, fabricated, and tested here. The attenuation effect is achieved by changing the curvature of a reflection mirror membrane. In order to demonstrate the proposed principle, concave mirrors with different radii are first used to confirm the attenuation effect. Then a thermal bimorph structure with membrane is used to deform the mirror membrane at different input voltages to show the variable attenuation effect. The size of the mirror membrane, which is made of low-stress Nitride (1.0 μm) with Cr/Au (0.4 μm) on the top surface, is 800 μm X 800 μm. The major two materials in bimorph actuator are low-stress Nitride and Al (1.2 μm). Two other materials, Poly-Si (0.4 μm) and SiO2 (0.5 μm), act as the heating and insulation layers in the bimorph beams, respectively. The minimum insertion loss of the attenuator is found to be 0.74 -dB. The attenuation and deflections of the mirror membrane at the central part are calibrated at different input voltages. It shows that and the attenuation is more than 40 -dB at input voltage of 44 V when the central displacement of the mirror membrane is about 16 μm.