Experimental studies on humidity measurements by using a FBG-based humidity sensor with multi-layer polyimide coating are demonstrated. As an important sensing application, the FBG humidity sensors have many unique advantages, such as immunity to electromagnetic interference and compact size, over the conventional electrical humidity sensor. In the detection principle, The Bragg wavelength of the FBG sensor shifts with the volume of the moisture absorbed by the polyimide film coated on the FBG. Experimental results show a good linear relationship between the shifts of the Bragg wavelength of FBG sensor and the changes in the relative humidity level. The preliminary experiments for investigating the performances of this polyimide-coating FBG humidity sensor were carried out and several experimental results in measurements of the relative humidity in air and moisture in oil are presented.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.