The present invention provides a magnetic monocomponent developer which can obtain a stable conveying performance for a long period and an image forming method which uses the developer. In a magnetic monocomponent developer which is used in a developing unit which includes a developing vessel which accommodates a magnetic monocomponent developer therein, a developer carrying body which carries the magnetic monocomponent developer and conveys the magnetic monocomponent developer to a developing region, and a developer layer thickness restricting member which restricts a layer thickness of the magnetic monocomponent developer, the magnetic monocomponent developer includes toner particles which contain at least a binder resin and magnetic powder, and also includes following constitutions (a) to (c). (a) an average particle size of the toner particles may being set to a value which falls within a range from 6 to 9 .mu.m. (b) an average degree of circularity of the toner particles being set to a value which falls within a range from 0.950 to 0.960. (c) a content of the toner particles having a degree of circularity of below 0.850 being set to a value which falls within a range from 2.0 to 4.0 unit % with respect to a total quantity of the toner particles.