Abstract: A strain of Antagonistic actinmycete T12 with bacteriostatic effects on several kinds of pathogenic bacteria was isolated from soil sample. And it was preliminarily identified by morphological characteristics, biological and physical characteristics. The stability of its fermentation liquid was determined. The results showed that T12 had better antibacterial effects on Gram-positive bacteria. At above 70 ℃, the antibacterial activity of the fermentation liquid was slightly weakened with the rise of temperature. And it still showed some antibacterial activity at 100 ℃. The fermentation liquid had stronger stability to temperature. When pH was lower than 6, the antibacterial activity of the fermentation liquid decreased with the reduce of pH, but it showed stronger stability to alkali.