A developer for developing an electrostatic image has a toner which contains a particulate toner, a particulate silica A and a particulate silica B. The toner has a weight-average particle size of not larger than 12.0 .mu.m and a particle-number distribution showing not more than 50% of toner particles not larger than 4.0 .mu.m in particle size and not more than 10% of toner particles not smaller than 10.08 .mu.m in particle size, each of the toner particles having a toner composition containing at least a polymer component and a charge controlling agent. The particulate silica A is composed of silicone oil-treated silica particles and has an average particle size of not larger than 0.1 .mu.m, while the particulate silica B is composed of silicone oil-treated silica particles and has an average particle size of 0.5 to 50 .mu.m. The particulate silica B also has a particle-number distribution showing not more than 50% of silica particles not larger than 1.0 .mu.m in particle size and not more than 10% of silica particles not smaller than 100 .mu.m in particle size. Also disclosed is an image forming method which uses this developer.