Abstract: The characteristics of an endophytic bacterium (X4), isolated from leaves of Kobresia capillifolia in the East Qilian Mountains Alpine grasslands, were studied using Salkowski colorimetric analysis, the phosphorus cycle and confronting cultivation. Secretion of IAA was 50.74 mg/L in King’s medium containing tryptophan. Its dissolved phosphorus circle diameter/bacterial colony diameter (D/d) values were 4.31 and 3.23 respectively. The bacteria inhibited pathogenic fungi, such as Rhizoctonia solani, Sclerotinia sclerotiorum, Botrytis cinerea, and Fusarium oxysporum, and led to abnormal mycelial growth of these fungi. The pH range of colony growth was 5-10 and resistance to NaCl solution was less than 7%. Thalli were short rods, and Gram-negative with no spores. The 16S rDNA gene sequence of X4 was most similar to Serratia plymuthica. The bacterium has the potential to make microbial fungicides and to improve soil.