In this paper, we tried to make clear the effects of spray characteristics, especially spatial inhomogeneity of fuel droplets, on ignition probability of fuel spray. A mono-dispersed methanol spray was ignited by a discharge spark ignition system, and a large number of ignition tests were carried out. As a result, ignitability of fuel spray is affected not only by the instantaneous spray concentration just before ignited, but also by other probabilistic factors based on an inhomogeneity in spray. The ignition process can be classified into two processes, i.e. flame generation and flame propagation. The flame propagation process becomes major control factor of the result, as the discharged energy is sufficiently large. Further, the propagation process is strongly affected by inhomogeneity. Therefore, inhomogeneity of spray was expressed quantitatively by the inhomogeneity index and the relation between inhomogeneity of spray and ignitability was discussed. Results suggest that there is the optimum intensity and the scale of inhomogeneity for ignition. In this research condition, the characteristic scale of inhomogeneity was around 15mm for successful ignition and the both of the characteristic scale and its intensity of successful case were smaller (or less) than those of failed one.