Large outdoor fires are one of the prominent fire problems in the world. Spot fires, caused by firebrands, are known as a key mechanism of rapid fire spread. Firebrands ignite unburned fuels far ahead of the fire front. In large outdoor fires, firebrands are thought to accumulate and ignite unburned fuel which cannot be ignited by a single firebrand. Experiments are performed to investigate the ignition behavior of fuel beds with changing fuel moisture content (FMC) and wind speed. It was found that accumulated firebrands could cause ignitions on fuel beds with high FMC and the upper limit of FMC for ignition increased from 6 m/s to 8 m/s.