A new strain sensor with a compact structure has been designed, demonstrated, and characterized. The strain sensor composes of a pair of adjacent cantilever beams with different lengths connected by a short tip beam. Residual strain causes two beams to deflect and to magnify the deflection into optically measurable displacement of the tip beam. The analytical model to relate the measured displacement to residual strain has been derived, and its results are then verified by finite element modeling. The strain sensor does not need correction factor for the strain-displacement relation. The relation of the displacement and residual strain is linear, and the displacement is independent of both Young’s modulus and the thickness of the film. All three properties are not found simultaneously in previously reported strain sensors. Based on the same strain with the same calibrated displacement, our structural size can be smaller than other strain sensors. The strain sensor has been fabricated and demonstrated by using a wet SiO2 film, and undoped LPCVD polysilicon films without post annealing.