A model for examining ignition of a single fuel droplet passing through a propagating flame was presented, and the effect of the initial droplet diameter and the equivalence ratio of the mixture on ignition time of a droplet was investigated numerically. The results showed that each equivalence ratio had a minimum ignition time against the initial droplet diameter because of the competition between the evaporation time and the reaction time, and that each initial droplet diameter had a minimum ignition time against the equivalence ratio of the mixture because of the competition between the effect of the flame temperature and that of the oxidizer concentration. The results also showed that the burning velocity of propagating flame increased as the flame front approached the droplet surface. This increase of the burning velocity may result in a local disturbance on the planar propagating flame through flame-droplet interaction and thus suggests the occurrence of the thermal-diffusion instability in spray combustion.