In order to realize a lean-burn spark ignition engine, the promotional effect of non-thermal plasmas on flame propagation in air/fuel mixtures was investigated using a rapid compression and expansion machine (RCEM). Liquid fuels such as n-heptane, iso-octane, and toluene were tested. The non-thermal plasmas were generated by a dielectric-barrier discharge (DBD) installed in the combustion chamber near the spark plug. A part of the lean air/fuel mixture in the chamber was passed through the non-thermal plasma and transported to the position near the spark plug by induced flow based on its electrohydrodynamics (EHD) effect. Temporal growth of initial flame size was observed by a high-speed camera with an image intensifier. The dependence of the initial flame development time and flame speed at a 5 mm radius on initial temperature, equivalence ratio, and the kind of fuel were examined. It was found that flame propagation was promoted by combining the DBD and spark ignition for lean mixtures in certain initial temperature regions and fuels that have a large heat release in the low temperature oxidation region.