An experimental study of the liftoff heights and visible flame lengths of hydrogen jet diffusion flames in still air as well as the liftoff, blowout, and blowoff characteristics of pure and diluted hydrogen flames with or without co-flowing air stream is presented. The results show that away from the forced convection limit the flame lengths for different fuel nozzle diameters can be collapsed onto a single curve if they are plotted in terms of the non-dimensional groupings. The jet diffusion flame in still air is liftoff and drop-back at an exit velocity of 535m/s and 320m/s, respectively. As the co-flowing air and pure hydrogen exit velocities are respectively increased to 7.5 ∼ 8.5 m/s and 500m/s, a very sensitive blowout region appears in the diffusion flames. Further increases of the co-flowing air velocity beyond 8.5 mlscause the flames blowoff. Under no co-flowing air conditions, the blowoff of the hydrogen diluted with air flames is dependent on the mixture exit velocities and diluent concentrations, and is independent of nozzle diameters. However, the blowout of the diluted flames is dependent on the exit velocities, diluent concentrations, and nozzle diameters.