A numerical investigation is carried out to study the ignition of a hydrogen/air mixture by a hot turbulent jet. A stand-alone probability density function method is used to model the turbulent reacting flow. In order to reduce the computational cost of chemical reactions, a reaction diffusion manifold technique is applied for an automatic mechanism reduction. The ignition delay time and the ignition length are investigated with respect to various inlet temperatures as well as the nozzle diameter and the jet inlet velocity. The simulation results reveal that immediately after a discharge of the hot exhaust gas from the nozzle, the hot gas cools down due to the strong turbulent mixing with the cold ambient mixture. However, further downstream where the turbulent mixing intensity is reduced, the heat release due to chemical reactions overcomes the heat dissipation, which eventually leads to ignition.