High-pressure hydrogen leak is one of the top safety issues presently. This study elucidates numerically the physics and mechanism of high-pressure hydrogen jet ignition when the hydrogen suddenly spouts into the air through a tube. The direct numerical simulation based on the compressible fluid dynamics was carried out. when high-pressure hydrogen is passing through the tube filled by atmospheric air, a strong shock wave is formed and heats up hydrogen behind the shock wave by compression effect. The leading shock wave is expanded widely after the tube exit, auto-ignitions of hydrogen occur. When the tube becomes longer, the tendency of auto-ignition is increased. Other type of auto-ignitions is also predicted. An explosion is also occurred in the tube under a certain condition. Vortices are generated behind the shock wave in a long tube. There is a possibility of an auto-ignition induced by vortices. (c) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.