In vitro antidiabetic effects of seven Turkish and five Sri Lankan black tea (Camelia sinensis) samples collected from Turkish market were investigated through enzyme inhibition assays. Foreign matter, non-oxidized particles, water soluble matter, loss on drying and total ash values were determined and compared with the Turkish Food Codex-Tea Notification and found to be compliant with the standards. Total phenolic and flavonoid contents, caffeine, gallic acid and rutin amounts of tea samples were measured. Ultra-pressure liquid chromatography with time of flight mass spectroscopy (UPLC-TOF-MS) was used to compare the major compounds of tea samples. All tea samples had potent inhibition against alpha glucosidase (>100-97.86% at 3 mg/ml). Water soluble matters of Sri Lankan teas are higher than them of Turkish samples (except TL1). Caffeine amount of the Sri Lankan tea samples were approximately twice of Turkish black tea samples. Sixteen common compounds were identified in tea samples by UPLC-TOF-MS.