A novel liquid level sensor is proposed. A segment of thin-core single-mode fiber (TCSMF) is inserted in a single-mode fiber (SMF) by splicing to form an in-line Mach-Zehnder interferometer (MZI). The phase difference between cladding modes and core mode is changed with the change of liquid level, and then the transmission spectrum of MZI is changed. The principle and sensitivity of the sensor are analyzed theoretically, and experimental results show that the shift of the peak wavelength is almost linearly proportional to the change of liquid level and the sensitivity increases with the refractive index (RI) of tested liquid, which is content with the analyzed results. The sensitivities of 0.160 nm/mm and 0.228 nm/mm are obtained in pure water and saturated salt water, respectively. The sensor is characterized by full optical fiber structure, easy fabrication and high sensitivity, and it is promising in measuring accurately the level of liquid with refractive index less than that of cladding.