Abstract: The aim of these experiments was to investigate the effects of including oat hay in the diet on in vitro ruminal fermentation. Ruminal fluid was collected from six cross breed sheep (Merino×Mongolian) with similar body condition and body weight (70.32±2.14 kg). The three diets had different ratios of oat hay and corn silage (dry matter basis), i.e., C₁ (concentrated feed+100% corn silage), C₂ (concentrated feed+50% oat hay+50% corn silage) and C₃ (concentrated feed+100% oat hay). In all the diets, the ratio of concentrate to roughage was 35:65. The results indicated that: 1) adding oat hay to the diet significantly increased gas production and methane production (P<0.05). Gas production was slightly higher in the C₂ group than in the other groups, while methane production in the C₂ group was lower than that in the C₃ group but higher than that in the C₁ group. 2) Adding oat hay to the diet did not affect pH, but it significantly increased the NH₃-N content (P<0.05). The NH₃-N content was slightly higher in the C₂ and C₃ groups than in the C₁ group. Adding oat hay to diets did not affect the proportion of propionic acid, but significantly increased the proportion of total volatile fatty acids, acetate acid, and butyric acid, and the acetate/propionate ratio (P<0.05). 3) Adding oat hay to the diet significantly increased the in vitro dry matter degradation rate and in vitro protein degradation rate, and promoted degradation of rumen nutrients. 4) The highest multiple-factors associative effects index was in the C₂ group, followed by the C₁ group. In conclusion, in a 35∶65 concentrate: roughage ratio diet, the C₂ diet (1∶1) could improve the rumen environment, and achieve the maximum combined effect.