This experiment was conducted to determine the effect of feeding Soybean Oil (SO) along with vitamin E on performance and mammary gland health in Holstein cows. Eight Holstein lactating cows were employed in a 2×2 factorial experiment and in an incomplete replicated latin square design. Treatment diets were: 1) 0% SO plus 0 IU of vitamin E, 2) 3% SO plus 0 IU of vitamin E, 3) 0% SO plus 8000 IU of vitamin E, and 4) 3% SO plus 8000 IU of vitamin E. At the end of each period milk and blood samples were taken. Increasing dietary vitamin E increased milk fat. Soybean oil addition decreased milk fat production, and increased milk lactose. Neither soybean oil nor the interaction of SO by vitamin E had any effect on either milk Somatic Cell Count (SCC) or Total Microbial Count (TMC). Animals fed Vitamin E had lower SCC and TMC than animals not fed the vitamin. Results finally indicated that vitamin E supplementation at a high level reduces SCC while increasing mammary glands' health in lactating dairy cows.