Open AccessThis study was conducted to evaluate the effects of rosemary essential oil (REO) on feed digestibility, ruminal fermentation and blood metabolites of Ghezel sheep. Four male sheep with average body weight 46 ± 2.0 kg were used in a 4×4 Latin square design. Treatments were control (no REO added), 100 mg d-1 of REO (low), 200 mg d-1 of REO (medium) and 400 mg d-1 of REO (high). Sheep were fed the 4 diets for 4 periods of 21 days (14 days as adaptation and 7 days for sample collection). The results showed that digestibility of dry matter, neutral-detergent fiber, acid-detergent fiber and crude protein were not affected by REO feeding (p>0.05). The concentration of ammonia-N across sampling times was lower (p<0.05) at low REO dosage compared with control. The molar proportion of acetate and butyrate across sampling times were lower at low REO dosage compared with control (p<0.05). Total volatile fatty acids (VFA) concentrations at 4 h after morning feeding were reduced (p<0.05) by adding 100 mg of REO d-1 to diet compared with the control, whereas medium REO dosage increased (p<0.05) total VFA concentrations at 4 h post feeding compared with the control. The addition of REO had no effect on total protozoa counts across sampling times (p>0.05). Supplementation with REO had no effect on plasma concentrations of glucose, triglyceride, cholesterol, total protein and albumin (p>0.05). The results of this study indicate that, although a medium dose of REO may have positive effect on rumen fermentation, a low dose of REO may have adverse effects on ruminal fermentation.