The paper presents a thermopneumatic microactuator that utilizes the slope-deflection of a pressurized circular diaphragm to obtain a long-stroke actuation in millimeter range. In a theoretical study, the load-deflection relationship and the stress-deflection relationship of the circular flat diaphragm are analyzed using an equivalent circuit model of the thermopneumatic actuator. A tantalum resistive hearer and electrical interconnection pads are fabricated on a glass plate by surface-micromachining technique and the other components are manufactured by conventional mechanical machining techniques, including milling, sand blastering, and electrical discharge machining. The fabricated microactuator generates a 400μm deflection of the diaphragm and 2 slope-deflection of the drive-plate, which in turn produces a 1.2mm stroke at the end of the drive-plate. A set of design variants having three different resistive heaters, has been also fabricated and tested for the power supply of different voltage levels.