In the accident of Fukushima Daiichi nuclear power plants, hydrogen was accumulated in the reactor buildings and exploded. To prevent such explosions, hydrogen venting from reactor buildings is considered. When the gas mixture is released to a reactor building through a reactor containment, together with the hydrogen some amount of steam might also be released. The steam condenses if the building atmosphere is below the saturation temperature, and it affects the hydrogen behavior. In this study, the condensation effect to the hydrogen venting is evaluated using CFD analyses, by comparing the case where a hydrogen-nitrogen mixture is released and the case where a hydrogen-steam mixture is released.