The autoignition characteristics of a binary component fuel droplet inside a rapid compression machine (RCM) were investigated. An RCM is an experimental setup for simulating a single compression stroke of an internal combustion engine. During the experimental time, the temperature and pressure inside the reaction chamber were temporally varied, and the ignition of a binary component droplet occurred when sufficient fuel vapor was generated around the droplet. n-Heptane was selected as the base fuel, and a specific volumetric ratio of iso-octane and n-decane was added as fuel additives. The droplet was placed at the center of the reaction chamber by suspending at the tip of a thermocouple, and its transient temperature was measured. The initial droplet diameter was in the range of 450-1000 mu m, and the temporal variation of droplet diameter was measured by analyzing images from a high-speed charge-coupled device array camera. In the single component experiments, a monotonic increase of the ignition delay with droplet diameter was observed, and the gradient was different for n-heptane and n-decane. The iso-octane droplet did not ignite in the present experimental conditions. The addition of iso-octane in n-heptane increased the ignition delay by dissipation of n-heptane vapor and an autoignition temperature change. The ignition delay and maximum droplet diameter ignition limit rapidly changed when the volume fraction of iso-octane was approximately 0.5, and the effect of iso-octane on the ignition delay was diminished for lower volume fractions. The ignition delay also increased with n-decane addition by suppressing the evaporation process. Compared to iso-octane, the increase in the n-decane volume fraction resulted in a gradual change of the ignition delay and maximum droplet diameter ignition limit due to the different mechanism of the ignition suppression process.