A miniature air-breathing twin-cell steak is designed and evaluated for direct formic acid fuel cell (DFAFC) applications. The stack consists of two face-to-face single cells with one shared fuel reservoir. This particular design has advantages in volume reduction relative to single cells in series connection. The performance, stability and reproducibility of the stack are investigated extensively for practical fuel cell applications. A maximum power density of 44.5mWcm(-2) is obtained with 5.0M formic acid solution as the fuel and Pt catalysts in electrodes. It is also found that the stack yields high stability and reproducibility when discharged at a constant current of 20mA. The output voltage can be maintained at 1.14V for about 5h feeding 3.5ml 5.0M formic acid solution and the performance can almost be reproduced when the fresh fuel is injected. (C) 2010 Elsevier B.V. All rights reserved.