Simulations of gas explosion of hydrogen/air mixture inside two rooms connected by ducts are carried out. Scalar transport chemical reaction model and LES turbulence. model are utilized to reduce the calculation load and to conduct real-scale analysis. The effects of ignition source locations and volume of ignited room are analyzed, and the time history of pressure and rate of pressure rise in each room are focused in this study. When the volume of the ignited room is larger than the other room, the high pressure from the other room causes a force to act on the partition to the ignited room. This study indicates that the current technique can predict specific features of gas explosions inside two rooms connected by the ducts. (C) 2007 Elsevier Ltd. All rights reserved.