In this Letter, a compact optical inclinometer in sub-centimeter size is proposed and demonstrated. A 1×1 mm2 GaN-on-sapphire chip composed of a light-emitting diode and photodetector is fabricated through wafer-scale processes and integrated with a spherical glass cavity with a diameter of 5 mm, which contains ethanol as a liquid pendulum. When applying inclinations relative to the horizon, the extent to which the chip is immersed in ethanol changes, thereby altering the amount of light received by the on-chip detector. The underlying mechanisms of angle-dependent reflectance characteristics at the sapphire boundary are identified, and the measured photocurrent signal can be used as quantitative readouts for determining the angle of inclination from -60 to +60°. A linear response with a sensitivity of 19.4 nA/° and an estimated resolution of 0.003° is obtained over a wide linear range from -40 to +40°. Verified by a series of dynamic experiments, the developed inclinometer exhibits a high degree of repeatability and stability, which paves the way for its widespread usage and applications.