This paper presents a scheme of designing finite-time high-order sliding mode (HOSM) observer which provides some essential requirements to be used in a sensorless control. The observer design technique is proposed to estimate some key states in a Multi-Input Multi-Output (MIMO) proton exchange membrane fuel cell (PEMFC) in a finite-time. Since variation of the load current deeply affects the life time of the cell, estimation and control of oxygen excess ratio (lambda(O2)) is suggested to detect and prevent the damage. As a practical application, the observer reconstructs oxygen excess ratio using measurable variables, such as the compressor angular speed, the supply, the return manifold pressures and the load current. The estimation is performed to keep stability without need of any transformation to a canonical form in a finite time. The designed finite-time observer is shown with some increases in the response time indices, improves the accuracy whilst guarantees a fast convergence with respect to using flow sensors. Simulation results verify the achievements whilst signify the fast response as well as robust against uncertainties and disturbances. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.