The autoignition characteristics of single n-heptane droplets inside a rapid compression machine (RCM) were investigated. During the compression stroke, the temperature and pressure inside the reaction chamber both rise rapidly, and subsequently decrease after the piston reaches top dead center. When a fuel droplet experiences these transient conditions, if sufficient vaporization occurs, the droplet can autoignite. A single n-heptane droplet was placed at the center of the reaction chamber. The droplet was suspended from the tip of a 50-mu m-diameter thermocouple, and its transient bulk temperature was measured. The evolution of the droplet was recorded using a high-speed charge-coupled device array camera with a frame rate of 500 fps. The initial droplet diameter was in the range of 400-1000 mu m, and the compression rate was varied; compression stroke durations of 185 ms and 235 ms were investigated. The ignition delay was longer when the initial droplet diameter was larger due to the slower heating of the droplet. Ignition did not occur if the droplet exceeded a critical diameter because sufficient fuel vapor was not generated during compression. The ignition delay was shorter for the 185-ms compression stroke owing to the droplet evaporation dynamics.