The present work reports a simple theoretical model to calculate the effect of the particle size on the activation energy and the ignition temperature of metallic nanoparticles. The activation energy was deduced from the particle cohesive energy and the ignition temperature was calculated using the condition that the heat generated by the combustion reactions is sufficient to counterbalance the particle heat loss to the surrounding. Heat loss was assumed to be in the transient regime and the combustion heat generation was calculated using the simplest Arrhenius-type model. Using aluminum as an example, the results showed that for particles of sizes larger than 50 nm, increasing the particle size had a little effect on the number of the surface atoms, the activation energy and the ignition temperature. As the particle size decreases the number of the surface atoms increases and the corresponding activation energy. E(d)/E(infinity) and the ignition temperature decrease. As the particle size decreased to about 5 nm and smaller, the activation energy could reduce to 20% or 50% of the bulk value and an ignition temperature as low as 800 K was obtained from the calculation depending on the ratio of the coordination number. Published by Elsevier Inc. on behalf of The Combustion Institute.