Abstract: The study investigated the effects of the combination of Pennisetum purpureum and Pennisetum sinese in different proportion on goats rumen degradation rate by using in vitro gas production technique so as to provide theoretical basis and data support for scientific application of roughage resources in South China. According to the ratio of dry substance, P. purpureum and P. sinese were mixed in the ration of 100∶0, 75∶25, 50∶50, 25∶75, 0∶100 and divided into group Ⅰ, Ⅱ, Ⅲ, Ⅳ, Ⅴ respectively, which were cultured for 4, 8, 12, 24 and 48 hours by using in vitro gas production respectively. Three replicates in each time point. We stopped fermentation and determined liquid pH, NH₃-N concentration and the amounts of DM (dry matter), CP (dry matter) and NDF (neutral detergent fiber) in fermentation residue. The results showed as follows: 1) Combinations of P. purpureum and P. sinese influenced rumen pH and NH₃-N concentration while pH decreased significantly (P<0.05) as the increase of the proportion of P. sinese, which was significantly lower than single P. purpureum group (P<0.05); The concentration of NH₃-N firstly decreased and then increased (P>0.05) with their normal fluctuation range, which was beneficial to rumen fermentation. Rumen TVFA (total volatile fatty acids) concentration increased gradually with the increase of P. sinese ratio. Ⅲ, Ⅳ and Ⅴ was significantly higher than that of single P. purpureum group (P<0.05) in 8-48 h. 2) The degradation rate of DM, NDF and CP increased significantly with the increase of the proportion of P. sinese, which was significantly higher than single P. purpureum group (P<0.05), and 75% P. sinese group was higher than those of the other groups as the better proportion. 3) The combined effects of P. purpureum and P. sinese were positive. The maximum combination effect (P<0.05) of DM degradation rate and positive combination effect of NDF degradation rate was obtained in 50% P. sinese group; The maximum combination effects (P<0.05) on the degradation rate of NDF in 75% P. sinese group was obtained at 12 hours; The combined effects on CP degradation rate were positive in P. sinese group at 4 and 48 h. We concluded that the combination of P. purpureum and P. sinese may improve the degradation rate of DM, NDF, CP and positive combination effect while better proportion of P. purpureum and P. sinese was 25∶75.