In a process for producing hydrated iron oxide which comprises the steps of adding an aqueous alkali solution to an aqueous solution of a ferrous salt in an amount no more than a neutralizing equivalent amount with respect to said ferrous salt, oxidizing the resulting ferrous hydroxide containing suspension to produce the seed crystals of hydrated iron oxide and subsequently supplying an additional amount of alkali and oxidizing the hydrated iron oxide to grow, sulfurous acid or a sulfite is added to either the aqueous ferrous salt solution or the aqueous alkali solution or the suspension containing ferrous hydroxide before the oxidation of ferrous hydroxide starts. The resulting hydrated iron oxide may be used as a raw material to produce berthollide, maghemite or a cobalt-doped ferromagnetic iron oxide.