A new fiber-optic bending curvature sensor based on thin-core fiber modal interferometer is presented.In order to eliminate the environment effect on curvature measurement,one standard single-mode fiber with a segment of thin-core fiber is used for sensing,while the other without is used for reference.The insertion of thin-core optical fiber and single mode fiber core mode mismatch,resulting in high-order cladding modes are excited and interfere with the core mode to form an in-fiber modal interferometer.When a curvature applied to the thin-core fiber,the mode indices and the fiber length will change,and thus the transmission peaks will shift.Both ends of the fiber are connected to the translation stage.When adjusting the translation stage distance,the curvature of thin-core optical fiber is changed,leading to the transmission dip move to shorter wavelength.According to this distance,the bending curvature of the sensor can be measured.Experimental results show that this device has low loss,low cost and high sensitivity.