Thymol and its four isomers were identified and quantified in essential oil of Citrus kinokuni Tanaka peel. Three of them were identified as thymol, carvacrol, and 3-methyl-4-isopropylpheno, respectively. The other two components had the similar structure with the identified three components, and their probably structure were elucidated. The quantitation result demonstrated that the thymol and carvacrol had high content in the five ismomers.