The effects of a non-steady state plasma discharge on flammability limits and flame structure of air natural gas mixtures are investigated. As plasma power increases, flame structure is changed and flammable range is extended. In the absence of a visible flame, a higher hydrogen production is observed, revealing that the discharge is a source of molecular hydrogen. The reduction on hydrogen production inside the flammable range suggests a burning of hydrogen in the flame. (C) 2016 Elsevier Ltd. All rights reserved.