Abstract: In steam reforming, polymerization/cracking of reactant could take place in parallel with reforming reactions, which are important routes for coking and might form coke of distinct nature with that from reforming. This was investigated herein by conducting decomposition or steam reforming of ethanol over Ni/SBA-15 catalyst at 400, 550 and 700 ºC, respectively. The results indicated that decomposition of ethanol formed more abundant carbonaceous species bearing C=C at 500–600 ºC than that from steam reforming. These carbon-rich intermediates were precursors of coke, rendering production of more coke (mass ratio of coke to catalyst in decomposition versus reforming: 48.6% versus 28.2% at 400 ºC; 86.4% versus 57.8% at 550 ºC; 28.7% versus 8.7% at 700 ºC), due to lack of steam to oxidize them into aliphatic oxygen-containing species (i.e. CO2*, C=O and C-O-C). High concentration of carbonaceous intermediates in decomposition of ethanol formed more aromatic and thermally more stable coke than that from steam reforming. Coking was the most significant at 550 ºC in decomposition or reforming, because of accumulation of carbonaceous intermediates from cracking and low efficiency for their gasification. This formed coke of irregular carbon nanotubes or carbon nanobeads with abundant aliphatic structures. Enhanced gasification rate at 700 ºC diminished coking, forming carbon nanotube of smooth surface and higher resistivity towards oxidation.