Ignition and explosion from accidental leaks of large amount of flammables into a long, underground trench may result in extensive damage and fatalities. Mitigation of such leak and explosion is difficult although not impossible. Typical explosion suppression measures such as water spray and inert powder cannot be readily applied owing to underground location. In this work, a concept of suppression the flame propagation instead of explosion overpressure was proposed via injecting inert gas into the underground duct to segregate the flammable gases and suppressing the flame propagation throughout the duct. Experimental studies were done with a small pipe with a diameter of 0.043 m and a large pipe with a diameter of 0.49 m. Tests were carried in an ignition section containing propylene/air mixture near stoichiometric concentration and generating a peak flame propagation speed of approximately 100 m/s. The ignition section is connected to a section filled with an inert gas, another section with flammable mixtures, and finally a sufficient long, ambient section to accommodate flame propagation. The critical length of the inert gas section required for successful suppression of flame from igniting the flammable section is found to be 0.3 m for CO2 and 0.9 m for N2 in the large pipe and 0.2 m for CO2 and 0.3 m for N2 in the small pipe. Finally, application of the results in responding to large-scale leak into underground duct is discussed.