A series of different-sized Ni catalysts (4.5-45.0 nm) were prepared and used for methane autothermal reforming with CO2 and O-2 in a fluidized-bed reactor. It was found that the activity and stability of Ni catalysts depend strongly on the particle size and the operating space velocity. Small sized Ni is more active and stable at space velocity < 54,000 h(-1). Characterizations disclosed that methane decomposition rate decreases with the enlarging Ni particle size, and some of the surface carbons (derived from methane decomposition) are inactive in CO2 atmosphere. As the methane decomposition rate slows on larger Ni particles and at higher space velocity to ensure complete conversion of the oxygen, surface Ni will be gradually oxidized by remaining O-2, leading to Ni deactivation. (c) 2007 Elsevier Inc. All rights reserved.