Numerical simulation of hydrogen dispersion in a partially open space is presented in this paper. The transient behavior of hydrogen and the process of accumulation in the space are discussed. The effects on the hydrogen concentration distribution of changing the vent positions, vent conditions and surrounding atmospheric currents are also shown. Based on the results, we discuss the conditions for preventing the dispersion and accumulation of hydrogen in the space, and the importance of incorporating the relationship with the outside environment in the computation. (C) 2007 International Association for Hydrogen Energy.