Abstract: The pathological changes in breast tissue of several cows suffering from clinical mastitis were observed.The mast cells were mainly located around the blood vessels,acinus and gland duct of the breast tissues.The results showed that the physiological function of mast cells may be achieved by changing the micro-circulation through vascular reaction,and the released mediator entered the blood vessels and played a role.And the raised polymorphonuclear leukocytes dissociated from the blood vessels.The number of mast cells in various parts of the mastitis breast tissue was higher than that in normal breast tissue,which indicated that the number of mast cells increased with the intensifying pathogenetic conditions.