In this study, two Sideritis species, endemic to Turkey, S. niveotomentosa Huber - Morathii, S. brevidens P. H. Davis have been studied for their diterpenic compounds and the antioxidant properties. Eight known diterpenoids, which have ent-kaurene skeleton, were isolated from acetone and methanol extracts of these species. The structures of the isolated diterpenes were determined by using the NMR (H-1-NMR, C-13-NMR, COSY, HMQC, and HMBC) spectroscopy. The analysis of the phenolic compounds of the extracts was performed by high performance liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). Also the antioxidant capacity of the extracts was investigated namely by two methods; free radical scavenging and beta-carotene bleaching activities.