The aim of this study was to investigate the effect of gas flow rate on the gas production rate from n-dodecane using steam reforming in-liquid plasma. A steam reforming of n-dodecane was carried out within the reactor vessel which was connected to a waveguide, an aluminum rectangular tubes that guides the propagation of electromagnetic waves with minimum loss of energy. The liquid medium used for plasma generation was n-dodecane (commercial reagent). The tip of a single electrode was positioned in the bottom center of the reactor vessel for plasma formation. The produced gas flowed through an aspirator and was trapped and collected in a water filled container. The gas production rate was measured and its compositions were analyzed using a gas chromatograph. The gas production rate by plasma with steam feeding was 1.4 times greater than that by plasma without steam feeding. The hydrogen content of the gas produced ranged from 73% to 82%. The maximum energy efficiency, as indicated by the ratio of the enthalpy difference of the chemical reactions to the input energy, was approximately 12%. The maximum hydrogen generation efficiency obtained from experiments was up to 59% higher than the efficiency of hydrogen production from electrolysis of alkaline solutions as reported in literatures. The energy payback ratio of hydrogen (EPRH2）was also calculated in order to obtain the hydrogen production efficiency.