The flame propagation velocities (FPVs) of hydrogen enriched propane and air mixtures were investigated by using an assembled annular stepwise diverging tube (ASDT) system. The system was invented and developed to cover the entire laminar burning velocity range including hydrogen-air flames. The critical FPVs, which are less affected by the channel gap scale in the assembled ASDT system, are comparable to the laminar burning velocities. The critical FPVs can predict the laminar burning velocities of the hydrogen enriched propane and air flames.