Flame propagation characteristics of hydrogen-air mixtures were investigated using an improved annular stepwise diverging tube (ASDT). An infrared detectable camera was used to visualize flame behaviors. Flame propagation velocity (FPV) was examined using the variation of equivalence ratio (or premixing ratio) and flow rate. Conclusively, it was found that hydrogen-air flames have critical FPVs that are not significantly affected by the channel gap scales. The critical FPVs were measured over a range of equivalence ratio of 0.27 to 6.50, and the maximum critical FPV was approximately 285 cm/s at the equivalence ratio of 1.7. These values were comparable to the laminar burning velocities. Quenching distances were also measured using the identical ASDT system, and the minimum value was about 0.60 mm at approximately stoichiometric conditions.