This paper describes a series of thermal micro-actuators fabricated from polycrystalline silicon. These actuators use geometric constraints to amplify motion from thermal expansion. Compliant mechanism theory, particularly the pseudo-rigid-body model, was used to aid in the visualization of the device’s behavior. Nonlinear finite element analysis was used to analyze the actuators and refine the designs. The resulting mechanisms were fabricated and tested. Maximum deflections of over 20 μm were obtained with a total cycle time faster than 4 milliseconds. A bi-directional actuator capable of 6 μm displacements in both directions is also described.