An experiment has been developed to investigate the ignition characteristics of ultra-lean premixed H2/air mixtures by a supersonic hot jet. The hot jet is generated by combustion of a stoichiometric mixture in a small prechamber. The apparatus adopted a dual-chamber design in which a small-volume (1% of the main chamber by volume) prechamber was installed within a large-volume main chamber. A small orifice (nozzle) connects the two chambers. Spark initiated combustion inside the prechamber causes a pressure rise and pushes the gases though the nozzle, resulting in a hot jet that would ignite the lean mixture in the main chamber. Simultaneous high-speed Schlieren photography and OH* Chemiluminescence were applied to visualize the jet penetration and the ignition processes inside the main chamber. Hot Wire Pyrometry (HWP) was used to measure temperature distribution of the transient hot jet. A novel velocity measurement technique based on Schlieren PIV (SPIV) has also been developed to characterize the local flow field. Three nozzle geometries (straight, convergent and converging-diverging) have been studied to understand their effect on ignition probability and characteristics. The results show that a supersonic jet by using a converging-diverging nozzle can ignite leaner mixtures than the jet produced by a straight nozzle of the same throat area (e.g., the ignition limit is reduced to ϕ = 0.22, from 0.35). Additionally, infrared imaging and OH* Chemiluminescence indicated diamond shock structures in the supersonic jets and a high-temperature zone downstream the shocks. This high-temperature zone is likely the reason why the main-chamber flammability limit can be further reduced. Lastly, combustion instability becomes noticeable near the lean-limit conditions for all three types of nozzles, which affect the structural integrity of the combustion chamber. Two instable frequency modes, natural frequency of the combustor at 180 Hz and a higher mode at 2400 Hz were observed.