Abstract: Deliberate ignition of the flammable gas mixture is a promising hydrogen mitigation measure under severe accident. Approach based on σ criterion and λ criterion to evaluate flame acceleration (FA) and deflagration to detonation transition (DDT) was established, and hydrogen deliberate ignition at early and late stage was analyzed in a single close room. The results indicate that deliberate ignition is an effective way to remove hydrogen, depending on igniter location and first ignition time. The approach can be used to determine number, location and frequency of a safe igniter system for a given dry containment in nuclear power plant.