Experiments have been carried out on the effects of pre-vaporized fuel in the ambience on spontaneous ignition behavior of isolated fuel droplets. An isolated droplet of n-heptane or n-dodecane, which is sustained by a quartz fiber, is inserted into a high-temperature gas field including fuel in the electric furnace. Methane and n-heptane are selected as fuels of the ambience. The ambient pressure and the temperature are set to be 0.3 MPa and 700 K, respectively, in order to prevent the consumption of methane and oxygen in the ambience. The condition of the ambience including fuel is elucidated using a numerical analysis of chemical reactions. The results indicate that the ignition delay time increases due to methane included in the ambience. In the case of the ambience where n-heptane vapor is included, both n-heptane and oxygen are consumed by chemical reactions prior to the droplet insertion. The ignition delay time of n-heptane droplet in the reacted n-heptane and air mixture is larger than that in air and chemical reactions at the reduced oxygen concentration seems to be slow. The ignition delay time is shorter in the reacted n-heptane and air mixture than in an air with the same amount of oxygen as the reacted mixture.