An all-optical fiber temperature sensor fabricated by a femtosecond (fs) laser is proposed in this study. The fiber sensor is of a very simple structure: a trench that overlaps with the fiber core, which makes a two light paths structure inline the fiber where one is the remained fiber core and the other is the trench. Then, the two transmitted beams interfere with each other and form a Mach-Zehnder interferometer (MZI). The sensor sensitivity is 55 pm/°C at temperatures in range of 30-220 °C with a step of 10 °C. The fringe visibility of the all-optical fiber sensor is greater than 30 dB at 150 °C for interference dip wavelengths, and the attenuation band has a 3 dB line-width of about 0.8 nm. Copyright © 2011 American Scientific Publishers All rights reserved.