This study performs the numerical simulation of hydrogen dispersion in a partially open space with a single roof vent. The effects of various roof vent positions, leak positions, leak flow rates and exhaust flow rates on the forced ventilation of leaking hydrogen, are shown and discussed. Based on the results, a proper roof vent position and the disadvantage of ventilation with constant exhaust flow rates are established. To overcome the disadvantage, a new control strategy to change exhaust flow rates with the roof vent fixed at the proper position is proposed. First a plot is constructed to show acceptable exhaust flow rates to various inflow rates and leak positions. Assuming real-time sensing of hydrogen concentration and height-direction velocity, volume flow rates of leaking hydrogen are then estimated. Based on the estimated leak flow rates and hydrogen sensor information near the roof, control is conducted considering the plot of acceptable exhaust flow rates to various inflow rates and leak positions. The proposed method is validated against various leak positions, leak flow rates and leak modes. This paper proposes an innovative approach to sensing-based risk mitigation control of hydrogen dispersion and accumulation in a partially open space by forced ventilation. (C) 2010 Professor T. Nejat veziroglu. Published by Elsevier Ltd. All rights reserved.