A magnetic Fe oxide containing Co, and optionally, a divalent metal, in which the oxidation degree indicated by the following formula ranges from about 30% to 80%; ##EQU1## wherein y is the amount of the Co, plus the divalent metal if present, in atomic % contained in the magnetic Fe oxide, and R is the ratio of the divalent Fe ion to the total Fe ions;and a process for producing a magnetic Fe oxide containing Co, and optionally, a divalent metal, and having an oxidizing degree of about 30 to 80%, which comprises dehydrating and reducing goethite containing Co, and optionally, the divalent metal, to obtain magnetic containing Co, and optionally, a divalent metal, and oxidizing gradually the magnetite at a relatively low temperature of lower than about 100.degree. C.