Using a high-speed video camera with an image intensifier, observations have been made on the propagating flames in the vertex rings. From their intensified images, variations of the flame speed and the flame diameter along the vortex axis have been determined. Results show that there are four types of flame propagation, (1) steady flame propagation, (2) periodic flame propagation, (3) unsteady flame propagation, and (4) random flame propagation. The steady flame propagation occurs for the maximum tangential velocity V_>7 m/s (Re ≈ 7 m/s in this experiment. (3) The unsteady flame propagation is accompanied with strong flame acceleration and deceleration, while its mean flame speed from ignition to the end is almost equal to the maximum tangential velocity. The random flame propagation occurs in turbulent vortex rings. The flame speed changes randomly. The luminous zone is quenched shortly behind the flame and its mean flame speed becomes much lower than the maximum tangential velocity
