Abstract: This paper presents the chemical constituents of the essential oils of Thymus quinquecostatus Celak. in Shandong Sheng. By applying the following techniques: (a) fractional distillation, (b) column chromatography, (c) TIC, (d) GLC and (e) IR, 16 components have been separated and identified e.i. linalool, borneol, p-cymene, cineole, carvacrol and others. It is also found that this species in Shandong Sheng has two chemotypes: linalool-type and carracrol-type. In addition, the variations of content of major components of the oils in various months have been determined.