A magnetic one-component toner of the invention contains at least a binding resin and a magnetic powder, and has a volume average particle size of 6.0 to 9.0 .mu.m, a compressibility of 15 to 50%, and a mean roundness of 0.950 to 0.960. The magnetic powder has at its surface a phosphorus element of 0.10 to 0.50% by weight. This toner is suitable for use in an image forming apparatus provided with a developer carrier made of stainless steel, or an image forming apparatus provided with an amorphous silicon photosensitive material. This toner has high transfer efficiency and less fog, thereby enabling to maintain image density for a long period of time, and form high image-quality image.