The impact of 46% Pt/C catalyst loading was investigated on mass activity and mass transport losses of proton exchange membrane fuel cell using experimental and simulation techniques. Our results indicated that, as the Pt loading decreased from 0.4 to 0.05 mg cm(-2), the mass activity increased by a factor of 2.7, but in the same time the mass transport resistance increased. To explain the observed phenomena, a novel catalyst layer model was suggested based on a self-assembled self-similar structure rather than randomly distributed spherical agglomerates. The utilized volumetric surface area, tha diffusion through the covering Nafion film, the Thiele modulus, and the gas phase diffusion were estimated using a novel fitting algorithm. The self-assemble nature of the catalyst layer was supposed to be characterized by its fractal dimension. It was measured independently by Hg-porosimetry, and found to increase from 2.7 to 2.93 as the Pt loading decreased from 0.4 to 0.05 mg cm(-2). The fractal dimension has been incorporated into the macrohomogeneous model, which resulted the observed increase of mass activity. Correlation is proposed between the fractal dimension and Thiele modulus, which accounts for the simulteneous deterioration of the catalyst layer performance at high currents and ultra-low loading of platinum. (C) 2013 Elsevier Ltd. All rights reserved.