Abstract: Previous studies have shown that endophytes can increase the resistance of drunken horse grass (Achnatherum inebrians) to many stresses, such as insects, diseases, waterlogging, drought, salt, and heavy metals. The aim of this study was to compare the resistance of endophyte-infected (E⁺) and endophyte-free (E^-) drunken horse grass to powdery mildew (Blumeria graminis). Forty pots of drunken horse grass (20 E⁺ plants and 20 E^- plants) were grown in a greenhouse. After 2 months, the natural infection rates, the percentage of diseased leaves, and the disease indexes of powdery mildew in both E⁺ and E^- plants were estimated and recorded. At the same time, the E⁺ and E^- drunken horse grass plants were scored as healthy, slightly diseased, or severely diseased based on a disease index. Then, four replicates of E⁺ and E^- plants were randomly selected to measure several physiological indexes; chlorophyll content, activities of superoxide dismutase (SOD) and peroxidase (POD), and the contents of malondialdehyde (MDA) and proline (Pro). Chlorophyll content was measured by acetone extraction, SOD and POD activities were evaluated using nitroblue tetrazolium and guaiacol assays, respectively, and MDA and Pro contents were determined using the sulfosalicylic acid and glucosinolate barbituric acid methods, respectively. The disease infection rate, the percentage of diseased leaves, and disease indexes of E^- plants were 97%, 86%, and 82.49, respectively, significantly higher (P<0.05) than their respective values in E⁺ plants (48%, 60%, and 50.76). The chlorophyll and proline contents and SOD and POD activities were higher in E⁺ plants than in E^- plants, regardless of whether the plants were slightly or severely infected. The MDA content was significantly (P<0.05) lower in E⁺ plants than in E^- plants. These results provide evidence that endophyte infection can increase the resistance of drunken horse grass to powdery mildew disease.