This work develops novel micro sensors to diagnose in-situ local performance in a fuel cell. The micro sensor is fabricated by micro-electro-mechanical systems (MEMS) technology. The substrate of the micro sensor is SS304 stainless steel with a thickness of 40 mu m. The micro sensor is flexible and low-cost, and has excellent mechanical properties. The variations in in-situ performance reveal significant local voltage variations in the under-the-rib area owing to more serious water flooding and fuel starvation than in the under-the-channel area. However, when the cell is operated at a lower relative humidity, liquid water that is stored in the under-the-rib area hydrate the membrane, increasing the local voltage. This work develops an in-depth fundamental understanding of local transport phenomena in a proton exchange membrane fuel cell (PEMFC). Novel micro sensors were designed and fabricated. The effectiveness of these novel micro sensors is demonstrated. They may also be applied to other applications and fields of research to obtain detailed local data to about fundamental phenomena. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.