The objective of this study is to determine essential oil, diterpenoid and phenolic contents of Sideritis hololeuca along with their biological activities. Phytol was found to be the main component of the essential oil. Seven known kaurane diterpenoids, siderol (140 mg, 0.16%, w/w), 7-acetoxy sideroxol (15 mg, 0.02%, w/w), eubol (6 mg, 0.01%, w/w), eubotriol (5 mg, 0.03%, w/w), 7-epicandicandiol (3 mg, 0.02%, w/w), ent-7 alpha-acetoxy-18-hydroxykaur-16-ene (5 mg, 0.01%, w/w) and linearol [by liquid chromatography-tandem mass spectrometry (LC-MS/MS)] were determined from the species. Moreover, vanillin (21 mg, 0.10%, w/w), which was not isolated from any Sideritis species earlier, was isolated from an acetone extract of S. hololeuca. Quantitative amounts of some phenolic compounds in n-hexane, dichloromethane, acetone, methanol extracts and infusion and decoction of the plants were also investigated by LC-MS/MS. Antioxidant capacity and acetylcholinesterase, butyrylcholinesterase inhibition effects of the species were evaluated. The extracts of methanol and infusion and decoction of species showed moderate butyrylcholinesterase inhibition activity. The highest inhibition was observed from the decoction of species. The Cu2+ reducing power of infusion was determined as 1.435 mmol TR g(-1).