Hydroxyl radicals produced in Fenton reaction can react with dimethyl sulfoxide to produce methane sulfinic acid which can further react with Fast Blue BB salt to form light yellow diazosulfone. According to the principle, hydroxyl radicals can be indirectly assayed by colorimetry. The determination condition was studied to obtain a best experimental program. The results showed that the hydroxyl-radical-scavenging effect were dose dependent upon thiourea and ascorbic acid. The walnut and black sesame were proved to be natural anti-oxidation foods. The method can be applied to sieve the hydroxyl radical scavenger.