It is expected that hydrogen will serve as a nonpolluting carrier of energy for the next generation ofvehicles, and guidelines for its safe use are required. Hydrogen-gas service stations for supplying fuel-cell vehicles will have to handle high-pressure hydrogen gas, but safety regulations for suchinstallations have not received much investigation. In this study, we experimentally investigated theflame characteristics of a rapid leakage of high-pressure hydrogen gas. A hydrogen jet diffusion flamewas injected horizontally from convergent nozzles of various diameters between 0.1 and 4 mm atreservoir over pressures of between 0.01 and 40 MPa. The sizes of the flame were measured, andexperimental equations were obtained for the length and the width of the flame. Flame sizes dependnot only on the nozzle diameter, but also on the spouting pressure. Blow-off limits exists and aredetermined by the nozzle diameter and the spouting pressure. Furthermore, the radiation from ahydrogen flame can be predicted from the flow rate of the gas and the distance from the flame.