The suppression phenomena of a nonpremixed flame stabilized by a recirculation zone behind an obstruction in a combustion tunnel have been studied because of their relation to fires in aircraft engine nacelles, dry bays, and shipboard and ground-vehicle compartments. The JP-8 fuel was supplied as a liquid fuel pool, or a gaseous (methane or ethane) fuel issued from a flat porous plate. downstream of a backward-facing step or J-shape flange. The OH planar laser-induced fluorescence revealed a narrow. wrinkled diffusion flame zone that was deeply folded into the recirculation zone in response to the dynamic air entrainment and back-flow. Both transient and steady-state fire suppression limits were determined by impulsively injecting a gaseous fire-extinguishing agent (nitrogen, CF3Br, or C2HF5), or by continuously adding nitrogen, into the approaching airflow in the combustion tunnel. For high air velocities, the normalized critical agent mote fraction at suppression was a unique function of the agent injection period normalized by the effective mixing time in the recirculation zone, independent of types of fuels, agents, and obstacles. The extinction of the relatively low-strain-rate diffusion flame zone in the recirculation zone appears to determine the suppression limits of obstruction-stabilized flames.