Abstract: The study was conducted by using in vitro gas production technique to investigate the fermentation characteristics of a compound forage mixed by silage of corns and rice straws(50∶50)and different proportional gramineae grasses.The results show that the maximum theoretic gas production,rate of gas production and lag time of gas production present different changing patterns,respectively,with the variation of mixed proportion.The obvious positive associative effects are noted when the mixture of rice straw and silage corn is mixed with Ponnisetum hydridum or Lolium perenne with the proportion of 25∶75.The significantly(P＜0.001) positive relationship is observed between gas production and content of crude protein or neutral detergent soluble matter,and the significantly(P＜0.001) negative relationship is found between gas production and content of neutral detergent fiber or acid detergent fiber.It can be concluded that adding pastures rich in fermentable carbohydrate and/or digestible protein into low-quality forage is essential and necessary to improve utilization efficiency of crude forage.