This study was performed to evaluate the effect of adding a mixture of peppermint, thyme and rosemary essential oils (EOs) in a ratio of (3:1:2), respectively, to the diet, on growth performance, rumen and blood parameters of fattening lambs. Twenty-one Mehraban lambs with an average weight of 29.80 kg were randomly allocated to one of three diets: 1) control, 2) base diet+1.25ml of EOs and 3) base diet+2.5ml of EOs. Daily weight gain was greater in the groups receiving the EOs than in the control, although this increase was mainly due to the interactions of treatment over time. Feed consumption and feed conversion ratio compared to the control did not altered. The total concentrations of VFAs and acetate in the groups receiving EOs decreased (P<0.05), while the concentration of propionate increased (P<0.05) at the level of 2.5ml of EOs and isovalerate increased (P<0.05) in both levels of EOs compared to the control. Total population of protozoa, rumen pH and ammonia nitrogen concentration were not affected by treatments. Glucose concentration increased at high level of EOs (P<0.05). Concentrations of urea, cholesterol, triglycerides, creatinine, total protein and albumin did not altered. The number of red blood cells, hemoglobin, hematocrit, number and percentage of white cells including lymphocytes and eosinophils were not affected by treatments but the percentage of monocytes decreased (P<0.05) at low level of the EOs. The results of this experiment showed that the addition of mixture of EOs to the diet did not have a significant effect on performance parameters and immune system of lambs, although it reduced the total concentration of VFA and acetate and increased the concentration of propionate.