Effect of glycerol in catalyst ink on the micro/nano structures of polymer electrolyte membrane fuel cells (PEMFCs) catalyst layers was investigated. Catalyst layers were fabricated by the decal method for various mass ratios of glycerol to carbon in the catalyst ink r(gc) (=m(g)/m(c)) from 0 to 20, and thus change the micro/nano structures of catalyst layer. The structures were evaluated by measuring the pore size with a mercury porosimeter and by observing the surface with a field emission scanning electron microscope (FE-SEM). At r(gc) < 5, although the residual glycerol was almost zero, the cell performance decreased with decreasing r(gc), suggesting that the structure of the catalyst layers significantly depended on r(gc). At r(gc) > 5, the remaining glycerol was proportional to r(gc). Because remaining glycerol blocks the pores and reduces the reaction sites, the cell performance decreased with increasing r(gc). If the remaining glycerol is removed by steam water, the performance is improved to the same level of the maximum performance for not steamed MEAs. (c) 2006 Elsevier Ltd. All rights reserved.