Abstract: Acorn contains variable amounts of tannins, so that causes occasional livestock toxicity. Because of its cheapness, accessibility and bad economic condition of many farms, oak ration is used in many parts of Iran. The 20 day period experiment was conducted on 9 female sheep (one-year-old and 40±3 kg weight) of the Karakul breed. Sheep were randomly divided into treatment group (n = 6) and control group (n = 3). In the treatment group, the mean amount of acorn powder added to control ration was 2.2 kg day-1. Venous Blood and urine samples were taken on 0, 10th and 20th days of experiment. At the end of experiment all the animals were slaughtered and histopathological samples were taken after necropsy. Then hematocrit and hemoglobin, serum glucose, total protein, albumin, fibrinogen, blood urea nitrogen, aspartate aminotransferase, Urine glucose and protein were measured. The results indicated that serum fibrinogen of treatment group increased significantly (p< 0.05) on 10th day. Other parameters didn`t show significant changes. Only mild hepatic fibrosis, lymphocytic hepatitis and interstitial nephritis were observed in one case of treatment group. It was concluded that the gradual increase of acorn powder in diet cause no overt clinical signs of oak poisoning in sheep.