Weak flames of four alkenes (ethylene, propylene, 1-butene and 1-pentene) were observed using a micro flow reactor with a controlled temperature profile to investigate their combustion and ignition characteristics. Weak-flame based investigation which enables to elucidate general ignition property of each fuel was conducted. One-dimensional computations with detailed reaction mechanisms were conducted to compare and analyze experimental results. Single luminous zone of hot flame was observed for all four alkenes. The order of weak flame position was ethylene, 1-pentene, 1-butene and propylene from the lower temperature side and thus the reactivity of those fuels are higher in the same order. Computational results captured the order of the experimental weak flame position. Alkene results were compared with alkane results in terms of carbon number and the order of weak flame position agreed between the alkene and alkane results. Weak flame structures of alkenes were analyzed using the computations. A similar overall flame structure was obtained for the four alkenes from the mole fraction profiles of major species. Rate of consumption was investigated to clarify fuel consumption of alkenes. Reactions with O, H and OH radicals proceed in the fuel consumption of alkenes. H-atom addition reaction of alkene where the reaction occurs at the double bond of the alkene is a unique reaction compared with the alkane and consumes a significant amount of the fuel. H-atom abstraction reaction with OH, which is important in alkane, is also important in the alkene consumption. Reaction path analysis was conducted to examine the high reactivity of ethylene. OH production through HO2 in the initial stage of oxidation is important against the weak flame position. Ethylene has a high rate of HO2 production compared with the other alkenes through HCO + O-2 <=> CO + HO2, C2H5 + O-2 <=> C2H4 + HO2 and C2H3 + O-2 <=> C2H2 + HO2, which results in a high reactivity. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.