To control the ignition possibility of hydrogen by electrostatic discharge at a ventilation duct outlet, we investigated the effect of the outlet shape. Four types of outlets were designed; outlet A (6.35 mm pipe), outlet B (12.7 mm pipe), outlet C (25.4 mm pipe) and a tapered porous outlet (called TP outlet in this paper). iron (III) oxide particles were used as the model dust. It was clarified that if the ventilation duct outlet is grounded, most of the electrostatic charges are not generated on the ventilation duct outlet. But, not all generation of electrostatic charges from the mixture of hydrogen and iron oxide can be removed by grounding only. However, the voltage and energy of the mixture at a certain downstream position can be reduced by using the TP outlet. This result means that the ignition possibility by electrostatic discharge may be reduced by using the TP outlet. (C) 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.