Abstract: A high sensitive refractive index (RI) sensor based on Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The sensing head is formed by a single-mode fiber (SMF) sandwiched between two short sections of thin-core fibers (TCFs), and the length of TCF-SMF-TCF MZI configuration is about 9 mm. A well-defined interference pattern is obtained as the result of fiber-core mismatch and core-cladding modes interference. The selected interference spectrum is extremely sensitive to the response of ambient RI. Using the proposed sensor to measure the glycerine aqueous solution with different RI, results indicate that the RI sensitivity of the sensor is 159 nm/RIU in the ambient RI range of 1.33 RIU～1.38 RIU. The fiber sensor has advantages of low-cost, simple mechanism, easy fabrication, high sensitivity, anti-electromagnetic interference, etc. It has great application potentiality in fields of bio-chemical measurement and environmental monitoring.