A refractive index (RI) sensor based on a cladding-etched thin-core single-mode fiber (TCSMF) sandwiched between two single-mode fibers is demonstrated.The experimental results show that the sensitivity,within the RI range of 1.333-1.340,is enhanced at least 6 times by etching.It increases with the surrounding RI and reaches 857.5 nm/RIU at RI of 1.3684,and it can be expected to be higher with the decrease of the cladding diameter of the TCSMF.