The effects of droplet diameter and liquid/gaseous fuel concentration on the ignitability of fuel spray are investigated. A large number of trials to ignite various fuel concentration mixtures of mono-dispersed methanol spray, evaporated fuel and air were carried out with a capacitive discharge spark ignition system. Ignition probability is defined as the ratio between the number of in success to that of all trials. As the results, liquid fuel spray helps to ignite leaner concentration mixtures than the lean limit of gaseous fuel only. If droplet diameter is held constant, the ignition probability increases gradually with increase of the liquid and gaseous fuel concentration. If the liquid fuel concentration is held constant, ignition probability increases with the decrease of droplet diameter until approximately 100 μm and decreases under 100 μm. This indicates the presence of the optimum droplet diameter for ignition.