Enthalpy-enhancing gas is used to optimize the ionic conductivity of atmospheric plasma-sprayed 3.9 similar to mol% yttria-stabilized zirconia (3.9YSZ) electrolyte. In the experiment, three hydrogen gas flow rates were used to control the plasma energy. The size of the 3.9YSZ feedstock powder was sieved to be in the range of 2045 mu m. When the hydrogen gas flow rate increased, the sprayed surfaces became rougher. Nevertheless, the lowest apparent porosity, the highest bulk density, and the hardest electrolyte were obtained at the hydrogen gas flow rate at 7 similar to L/min. A 3.9YSZ electrolyte with an ionic conductivity of 1480 mu (O cm)-1 was obtained at 800 degrees C, when the hydrogen gas flow rate was at 12 similar to L/min. It was controlled by the grain-boundary conductivity, which was closely related to the lowest migration energy, the highest density of the columnar grains, and the closest bonding between lamellar layers. The increase in the hydrogen gas could diminish the amount of the migration energy and increase the grain-boundary conductivity. On the other hand, a larger cubic lattice constant and a less amount of the monoclinic phase in the as-sprayed electrolyte helped obtain greater intragrain conductivity.