A toner for developing an electrostatic image is formed as a mixture of toner particles containing at least a binder resin and a colorant, and inorganic fine powder. The inorganic fine powder includes: (A) inorganic fine powder (A) treated at least with silicone oil, and (B) inorganic fine powder (B) comprising a composite metal oxide including at least Si as a constituent element and having a weight-average particle size of 0.3-5 .mu.m. Because of the inclusion of the two types of inorganic fine powders (A) and (B), the toner is stably provided with a high flowability and a high triboelectric charge under various environmental conditions including low-humidity to high-humidity conditions. The toner is suitably used in an image forming system including a contact-charging means, a contact-transfer means and a film (or surf)-fixing system.