Ignitability of fuel droplets, relevant to the performance of liquid-fueled combustor, is investigated. Explicit ignition criterion of a fuel droplet which is not only in an unbounded oxidizing environment but also in spray interior has been expressed. A general discussion has been conducted as far as the exponent of fuel concentration. Based on the analytical solutions in the limiting situations as well as numerical results in the literature, approximate ignition criteria have been expressed in explicit form in obtaining the expressions of the ignition criteria, both simplicity in the expression and applicability for the widely-varing parameters have been taken into account. Fair expressions have been obtained with 8% error at the most.