Abstract: The reaction that cobalt reacts with nitroso-R salt in pH 5.5～7 acetic acid solution and forms a red soluble complex was used in spectrophotometric determination of cobalt. The sample pretreatment and determination conditions, such as sample digestion method, chromogenic agent concentration and dosage, boiling time and storage time of the sample solution, etc were tested and optimized. The interference from elements of Fe, Cu, Ni and Mn on the determination of Co was also studied. The method has been applied to the determination of Co in Cu, Ni and Mn ores with satisfactory results.