The mechanisms of suppressing a laminar methane-air co-flow diffusion flame formed on a cup burner with water vapor have been studied experimentally and numerically. The methane burned in a steel cup surrounded by a glass chimney. A mist generator produced fine droplets delivered though the glass chimney with air. These droplets were heated into water vapor when they went though the diffuser. The extinguishing limit was obtained by gradually increasing the amount of water vapor to replace the air in the coflowing oxidizer stream. Results showed that the agent concentration required for extinguishment was constant over a wide range of the oxidizer velocity, i.e., a so-called "plateau region". The measured extinguishing mass fractions of the agents were: (16.7 ± 0.6)% for H 2 O, (15.9 ± 0.6)% for CO 2 , and (31.9 ± 0.6)% for N 2 . The computation used the Fire Dynamics Simulator (FDS) developed by the NIST. The numerical simulations showed that the predicted water vapor extinguishing limits and the flickering frequency were in good agreements with the experimental observations and, more importantly, revealed that the suppression of cup-burner flames occurred via a partial extinction mechanism (in which the flame base drifts downstream and then blows off) rather than the global extinction mechanism of typical counter-flow diffusion flames. And the flame-base oscillation just before the blow-off was the key step for the non-premixed flame extinction in the cup burner.cup-burner flames, water vapor, extinguishing limit, partial extinctionWith the phase-out of Halons, development of new fire suppressants has focused again on inert agents including gases (e.g. CO 2 , or gas mixtures), liquids (e.g. water mist) and foams (e.g. compressed-air foams) [1,2] . So far, water vapor, one of the principal combustion products and inactive species in flame, has not been seriously considered as a Halon replacement although it is an efficient inerting agent. This lack of interest in water vapor may be explained by the fact that water
