A new in-line Mach-Zehnder interferometer (MZI) sensor consisting of a stub of multi-mode fiber and an up-taper was proposed and demonstrated. Temperature measurement can be carried out by detecting wavelength shift. Dependency of sensitivity on interferometer length and dip wavelength was discussed. Experimental results showed a maximum temperature sensitivity of 113.6 pm/°C can be achieved, which is superior to most fiber temperature sensors based on in-line MZIs within the range from 20°C to 80°C, also a good mechanical strength can be obtained. The proposed sensor is a good candidate for temperature measurement, due to the advantages of simple structure, easy fabrication, costeffective and high sensitivity.