The objective of this study was to investigate the effects of increasing doses of carvacrol (C) and/or thymol (T) on the performance, rumen fermentation, and blood and carcass parameters of Merino sheep. Eighty-four 12-week-old male Merino lambs were randomly assigned to 7 treatment groups. The sheep were fed with the same concentrate mixtures including a control diet, carvacrol 100 mg/kg (C-100), carvacrol 300 mg/kg (C-300), thymol 100 mg/kg (T-100), thymol 300 mg/kg (T-300), carvacrol+ thymol 100 mg/kg (C-50 + T-50), and carvacrol+ thymol 300 mg/kg (C-150 + T-150). The C and/or T supplementation did not affect the feed conversion. The lambs fed with C and/or T diets had higher rumen pH, NH3-N, and total volatile fatty acid (VFA) compared to those in the control group. However, essential oil supplementation did not change the molar concentration of VFA. The serum urea and glucose in C and/or T groups were not found significant on days 0, 35, and 70 compared to the control group. Slaughter weights and other carcass parameters were similar between the groups. The effects of C and/or T supplementation on the rumen and production parameters showed limited effects when lambs were fed with the high concentrate diets.