We executed this experiment to observe side effects of warfarin, the anticoagulant that is used for preventing thrombus in cardiovascular surgery for calves. The 6 calves(70-130 kg) were used in this experiment regardless of sexes. We administered warfarin at 0.07 mg/kg daily for 25 days. Blood was collected before warfarin administration, every five days for 30 days. PCV, RBC, WBC, fibrinogen, total protein and platelet as blood test, prothrombin time (PT) as blood coagulation test, and AST, SDH, total bilirubin, BUN and creatinine as serum biochemical tests were measured. As the result of the experiment, PT has been gradually increased after warfarin administration. It has been gradually increased and remains within the therapeutic range from the third day to the 28th day. PCV and RBC were decreased significantly from the value before the administration (p < 0.05). In the serum biochemical test, SDH shows significant increase comparing the value before the administration (p < 0.05). AST and total bilirubin were increased gradually from the value before the administration. Considering the result of the experiment, to give wafarin to prevent thrombus in cardiovascular surgery, we can get anticoagulation effect with minimal administration(0.07 mg/kg, PO) from the third day of the administration. However because of the decreased PCV and RBC, it may cause anemia. It may cause damage to liver based on the result of serum biochemical test.