The present study aimed to estimate the effect of Mentha piperita essential oil on biochemical and haematological parameters of sheep. A total number of 16 rams (3-4 years old) were randomly distributed into four treatments based on the essential oil dose: control (T1), 10 (T2), 20 (T3) and 30 ml/kg (T4) dry matter. The serum biochemical and plasma haematological parameters measured in the study included blood urea nitrogen (BUN),  glucose, cholesterol, triglycerides, total protein (TP), albumin, globulin, white blood cell count (WBC) and red blood cell count (RBC). The results indicated that glucose concentration increased significantly (P<0.05) in T4 compared to the control. No significant difference was found in cholesterol, triglyceride, BUN, total protein, albumin and globulin between the control and treated groups. In addition, WBC was significantly (P<0.05) high in T3 compared to the control. We concluded that alfalfa and barley silage treated with Mentha piperita essential oil improved glucose and WBC count in rams.