Analyses of previous test data and theoretical analyses are carried out to investigate the influence of ventilation on fire development, toxic gas concentration, visibility and risk for fire spread to nearby vehicles, in a road tunnel without and with a water-based fire suppression system. In a tunnel without a fire suppression system, a slightly lower ventilation velocity is preferred to slow down the fire growth at the initial stage of evacuation. At the fire fighting stage the ventilation velocity can be adjusted up to critical velocity. In a road tunnel with a fire suppression system, a slightly higher velocity is recommended to reduce the toxic gas concentration and increase the visibility downstream.