A Fully transient numerical model of spontaneous ignition of single fuel droplets was developed. A physical part of the model is one-dimensional, and therefore it can employ a detailed chemical reaction model, which is necessaiy to reproduce complicated spontaneous ignition process of hydrocarbon fuels. The model simulates an isolated droplet in an open ambient at constant pressure. It was verified through relevant microgravity experiments for relatively large (〜0.7mm) droplets, and was successful in quantitative reproduction of two-stage ignition behavior derived from low-temperature reactions. Then, ignition of relatively small (<100μm) droplets can be numerically observed. However, an isolated droplet in an open ambient that is smaller than a certain initial diameter does not ignite unlike droplets in a spray. The model was modified to handle a droplet in a closed cell so that single droplets could be compared to sprays. Two-stage ignition behavior was observed even for such fine droplets. There was transition from heterogeneous ignition to homogeneous ignition with decreasing initial droplet diameter.