To make "Hydrogen vehicles and refueling station systems" fit for public use, research on hydrogen safety and designing mitigation measures are significant. For compact storage, it is planned to store under high pressure up to 40 MPa at the refueling stations, so that the safety for the handling of high-pressurized hydrogen is essential. This paper describes the experimental investigation on the hypothetical scenario of dispersion and explosion of high-pressurized hydrogen gas which leaks through a large-scale break in piping and blows down to atmosphere. At first, to comprehend the behavior of the dispersion of high-pressurized hydrogen gas, the measurement device was proposed and the transient concentration distribution was obtained. The explosion experiments were carried out with changing t(ign) the time of ignition after the start of dispersion, PO the initial hydrogen pressure, and d the opening (nozzle) diameter. In the maximum case (d=10mm, P-0=40MPa, t(ign)=0.85s), the overpressure larger than 50 kPa was detected at 10 m away from ignition point. Through these experiments, it was clarified that the explosion power depends not only on the concentration and volume of hydrogen/air pre-mixture, but also on the turbulence characteristics before ignition. To enforce the analysis of explosion mechanism, numerical computer simulation about the same experimental conditions was performed and the calculated overpressure waveform agreed well with those obtained in the present experiments. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.