This work investigates the catalytic performance of nanocomposite Ni/ZrO2-AN catalyst consisting of comparably sized Ni (10-15 nm) and ZrO2 (15-25 nm) particles for hydrogen production from the cyclic stepwise methane reforming reaction with either steam (H2O) or CO2 at 500-650 degrees C, in comparison with a conventional Ni/ZrO2-CP catalyst featuring Ni particles supported by large and widely sized ZrO2 particles (20-400 nm). Though both catalysts exhibited similar activity and stability during the reactions at 500 and 550 degrees C, they showed remarkably different catalytic stabilities at higher temperatures. The Nil ZrO2-CP catalyst featured a significant deactivation even during the methane decomposition step in the first cycle of the reactions at >600 degrees C, but the Ni/ZrO2-AN catalyst showed a very stable activity during at least 17 consecutive cycles in the cyclic reaction with steam. Changes in the catalyst beds at varying stages of the reactions were characterized with TEM, XRD and TPO DTG and were correlated with the amount and nature of the carbon deposits. The Ni particles in Ni/ZrO2-AN became stabilized at the sizes of around 20 nm but those in Ni/ZrO2-CP kept on growing in the methane decomposition steps of the cyclic reaction. The small and narrowly sized Ni particles in the nanocomposite Ni/ZrO2-AN catalyst led to a selective formation of filamentous carbons whereas the larger Ni particles in the Ni/ZrO2-CP catalyst a preferred formation of graphitic encapsulating carbons. The filamentous carbons were favorably volatilized in the steam treatment step but the CO2 treatment selectively volatilized the encapsulating carbons. These results identify that the nature but not the amount of carbon deposits is the key to the stability of Ni/ZrO2 catalyst and that the nanocomposite Ni/ZrO2-AN would be a promising catalyst for hydrogen production via cyclic stepwise methane reforming reactions. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.