This study investigates the development of an in situ micro-temperature sensor, and analyzes the in-plane temperature distribution in a unit polymer electrolyte membrane fuel cell (PEMFC). To measure in-plane temperature distribution accurately, a polyimide-based micro-temperature sensor is designed and fabricated. The developed sensor is a resistance temperature detector with a flexible polyimide substrate. It exhibits high sensitivity and flexibility, and can be easily installed inside a cell. The sensor is sufficiently small to measure the temperature inside the PEMFC. After the sensors are calibrated, six sensors with one unit are inserted into the cell to measure in-plane temperature distribution. Six locations are chosen to represent the temperature distributions in the inlet, center, and outlet of the channel. The effect of inserting the sensor into the fuel cell is investigated by measuring the polarization curves with and without the sensor. A 3D computational fluid dynamics (CFD) fuel cell model with the same geometry and electrochemical properties as those of the PEMFC is also developed and analyzed to compare in-plane temperature distribution with the experimental results. A 3D CFD-PEMFC model accuracy can be obtained by comparing the temperature distribution results with the experimental results. (C) 2014 Elsevier Ltd. All rights reserved.