A refractive-index sensor with micro-structured defect was proposed and demonstrated,and its refractive-index characteristics were studied experimentally.After inserting a segment of thin-core fiber into a standard single-mode fiber,high-order cladding modes were excited and interfered with the core mode to form an in-fiber modal interferometer because of the mismatch between thin-core fiber and standard single-mode fiber .By decreasing of the cladding-diameter of the thin-core fiber,the evanescent field of the cladding modes was enhanced,which improved the sensitivity of refractive-index.Experimental results show that the proposed device has advantages of low loss,low cost,high sensitivity and good linearity.