An electrothermally-activated polymeric stacked actuator is presented to generate a rectilinear in-plane actuation. The actuator has a novel composite design, comprising of SU8 thermal expandable polymer, a silicon heat conductor and a top thin-film aluminium heater. The silicon heat conductor has a symmetric meandering shape. It extends through a thick SU8 layer. Theory and numerical analyses shows that this design offers advantages of enhanced longitudinal thermal expansion and enhanced heat transfer across thickness of the insulating SU8 layer. It is effective in electromechanical actuation. An activated 530-micron long micro-machined thermal stack demonstrates a longitudinal stroke of 13 microns at 2V and 27mW, and a low temperature rise below 300°C. Therefore, it can potentially be used for wider applications where low temperature and power efficiency are concerned.