Agglomeration of aluminum nanoparticles (nano-Al) is inevitable due to the strong van der Waals forces. It is supposed to affect the combustion characteristics of nano-Al. However, the agglomeration effect is not fully understood yet due to the worse description of the morphological evolution of agglomerates during reaction. In this paper the morphological changes of agglomerated nano-Al, which mainly results from the oxide shell thickening when the particle temperature is below the melting point of alumina, are confirmed in the thermogravimetric analyzer (TGA) and Hencken experiments. Three factors (f(A),f(ht), and f(sp)) are defined to describe the morphological changes quantitatively through proposed formulas. Thus the agglomerate could be equivalent to an effective spherical particle to help studying the effect of agglomeration on combustion. The results indicate two combustion modes for different agglomerates. The small Al agglomerate is hard to combust fully due to the strong heat transfer with ambience, corresponding to the high-temperature oxidation mode. However, at the same environment, as the agglomerate becomes larger, the maximum particle temperature (T-p,T-max) increases until that the agglomerate burns fully, corresponding to the fully-fledged combustion mode. At that mode, the burning particle temperature is mainly controlled by the viscous sintering of agglomerates. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.