Abstract: Background and Objective: Bovine subclinical mastitis increases somatic cell count (SCC) without showing clinical symptoms, thus resulting in substantial economic losses to dairy farms. Numerous attempts to reduce SCC without using antibiotics have been reported but they have yet to be applied widely. This study investigated whether SCC can be reduced by feeding coffee ground silage containing large amounts of polyphenol, which is an inexpensive antioxidant to cows with subclinical mastitis. Materials and Methods: In cows with subclinical mastitis, the coffee ground silage feeding group was fed by adding 1.5 kg day–1 to normal feed and feed in the control group was not supplemented. Blood and milk were collected every week, the SCC in the quarter milk was measured and the oxygen radical absorbance capacity (ORAC) values in the blood and milk and the thiobarbituric acid reactive substances (TBARS) value in milk were measured. In order to investigate the immune kinetics, leukocyte subpopulation analysis was performed for 7 days after feeding. Results: Although the SCC did not show a significant decrease in the non-feeding group (n = 9), in the coffee ground silage feeding group (n = 11), it was decreased significantly (p<0.01) at the 5th week. Furthermore, ORAC values were significantly higher (p<0.05) in the coffee ground silage group (n = 5) than in the non-feeding group (n = 6), the TBARS value was significantly lower (p<0.01). In addition, bulk milk SCC after feeding was also observed in 150 milking cattle in the summer season under heat stress. Furthermore, in the subpopulation of leukocytes after 7 days of feeding, there was a significant increase in CD4+cells and CD14+MHC class II+cells on day 1 after feeding. Conclusion: These results suggest that feeding of coffee ground silage to cows with subclinical mastitis increased the antioxidant activity and immune activity, leading to a decrease SCC in milk.