This paper presents the design, simulation and characterization of a new type of in-plane thermal unimorph, which utilizes composite SU-8/silicon microstructures. The unimorph consists of a silicon skeleton and SU-8 photoresist, which encapsulates the silicon skeleton. The silicon skeleton is asymmetric in shape, consisting of a straight segment and a meandering segment. Gaps and surrounds of the skeleton are filled with the SU-8 polymer. Bonds between the polymer filling and the sidewalls of the silicon microstructure enhance thermal expansion and stiffness of the polymer in a transverse direction. Therefore, this unimorph design delivers excellent actuation performance. The composite actuator bends laterally when electro-thermally activated. A 530 µm long, 90 µm wide and 50 µm thick micro-machined device achieves a maximum lateral displacement of 25 µm at a 1.75 V driving voltage and at 17.8 mW input power, at an average temperature below 200 °C. It has a simulated lateral stiffness of 1.2 kN m⁻¹. At 1.75 V, it is estimated to produce a 30 mN lateral blocked force that is high compared to other micro-actuators.