A toner for developing an electrostatic image is constituted by at least a binder resin and a charge control agent. The binder resin has an acid value of 5-50. The charge control agent comprises an iron complex represented by the following formula: ##STR1## wherein X.sub.1 and X.sub.2 independently denote hydrogen atom, lower alkyl group, lower alkoxy group, nitro group or halogen atom; m and m' denote an integer of 1-3; R.sub.1 and R.sub.3 independently denote hydrogen atom, C.sub.1-18 alkyl or alkenyl, sufonamide, mesyl, sulfonic acid group, carboxy ester group, hydroxy, C.sub.1-18 alkoxy, acetylamino, benzoylamino or halogen atom; n and n' denote an integer of 1-3; R.sub.2 and R.sub.4 denote hydrogen atom or nitro group; and A.sup.+ denotes hydrogen ion, sodium ion, potassium ion or ammonium ion. The toner has a weight-average particle size (D.sub.4) of 4-9 .mu.m and including toner particles having a particle size of 5 .mu.m or smaller at 3-90% by number, toner particles having a particle size of 6.35-10.08 .mu.m at 1-80% by number and toner particles having a particle size of 12.7 .mu.m or larger at a percentage by volume of at most 2.0%, wherein the toner particles having a particle size of 5.0 .mu.m or smaller are contained at N % by number and at V % by volume satisfying a relationship:N/V=-0.05N+k,wherein k is a positive number in the range of 3.0-7.5.