The present paper reports the hazards of a hydrogen jet diffusion flame formed by high-pressure hydrogen gas. Hydrogen jets were horizontally released from circular nozzles with diameters ranging from 0.1 to 4 mm, and the release pressure varied from 0.01 to 40 MPa (gauge). The blow-off limits were determined from the nozzle diameter and the release pressure. The flame sizes were measured, and experimental equations were obtained for the length and width of the flame. The flame sizes depended not only on the nozzle diameter but also on the release pressure. In the case of slit nozzles, the flame length depended on the length of the shorter side of the slit nozzle. The radiation from the hydrogen flame could be predicted from the flow rate of the gas and the distance from the flame. (c) 2008 Elsevier Ltd. All rights reserved.