Abstract: To further evaluate clinical efficacy and safety of nano-silver breast injection agent in treatment of bovine mastitis,188 cows suffering from mastitis in clinic and 294 cows suffering from subclinical mastitis were selected from Hohhot City and Baotou City in Inner Mongolia area.Three batches of nano-silver breast injection agent were used for observation of clinical efficacy.The nano -silver breast injection agent of different batches did not have significant difference in treatment of clinical mastitis at the same degree but had significant difference in treatment of mild or severe mastitis（P〈0.01）.The nano-silver breast injection agent of the three batches had an average cure rate of 80.2% on mild clinical mastitis and an average cure rate of 71.4% on severe clinical mastitis.Their average effective rates were 92.7% and 83.1%,respectively.After the cows suffering from subclinical mastitis were treated with the same dose of drugs,average negative rate was 88.1% in nipple and 91.2% in milk area.These results showed that the nano-silver into breast agent had good therapeutic effects on clinical or subclinical mastitis.In addition,all the cows treated with the nano-silver breast injection agent did not displayed systemic or local allergic reactions and local irritation,which indicated its high security.