An experimental study has been performed on hydrogen jet diffusion flames from vertical circular nozzles burning in free air. The direct photographic method was employed to investigate the influence of fuel flow rate and nozzle diameter, d, on the flame morphology. Emphasis was placed on the determination of the flame length, L_f, over a wide range of the fuel flow rate. It became clear that (i) as the nozzle velocty is increased from zero, the flame burning under conditions of mixing with air is induced and its length is monotonically increased until a laminar-to-turbulent transition occurs, and (ii) after the transition takes place, the turbulent flame length is substantially independent of further increase in fuel jet velocity. A few relationships pertinent to the flame length, i.e. L_f versus u_j, L_f/d versus, Re, and L_f/d versus d, were proposed and examined. In the laminar diffusion flame case, effects of nozzle diameter and Reynolds number on the flame length appear if the experimental data are summarized using the dimensionless flame length, L_f/d. In contrast, a slight effect of the turbulent diffusion flame is observed. The hydrogen diffusion flames, for the burner tube of 1.24mm i.d., are found to be perfectly stable at a nozzle velocity of 300m/s.