In this study, hydrogen production by the in-liquid plasma methods reported thus far was compared, to find the optimal method and condition. Because the in-liquid plasma is a type of plasma generated within bubbles in a liquid, an ingredient with a small evaporation enthalpy (EE) is found to be more advantageous. In addition, it is necessary to select an ingredient with a high thermodynamic ideal efficiency (IE), as calculated using the enthalpy of the formation. The actual hydrogen production efficiency (HPE) of water (IE, 0.28 Nm(3)-H-2/kWh; EE, 44 kJ/mol), methanol (IE, 1.26 Nm(3)-H-2/kWh; EE, 38 kJ/mol), n-dodecane (IE, 2.99 Nm(3)-H-2/kWh; EE, 62 kJ/mol), were found to be 0.02, 0.28, and 0.13 Nm(3)-H-2/kWh, respectively. The highest HPE was obtained for the methanol decomposition, indicating that ingredients with low EE and high IE are advantageous for hydrogen production. Moreover, the HPE reduced because most of the energy of the plasma diffused to the surroundings. Therefore, it is essential to develop efficient heat recovery methods and heat insulation systems.