Abstract: The experiment investigated the associative effect of whole corn silage (MCS)- peanut vine (PV) and Leymus chinensis (LC). First, WCS was mixed with PV in the proportions of 100∶0, 90∶10, 80∶20, 70∶30, 60∶40, 50∶50, 40∶60, 30∶70 20∶80, 10∶90 and 0∶100. After the best combination of WCS and PV (BWP) was determined, the BWP was grouped with LC under these 11 proportions. Rumen fermentation in vitro was used and single factors associative effect indexes including the associative effects of gas production (AEGP), microbiological crude protein (AEMCP), dry matter degradation rate (AEDMD) and multiple factors associative effect indexes were analysed over 24 h. 1) Under the WCS-PV combination of different proportions, both AEGP and AEDMD were the largest at a ratio of 70∶30 and they were 0.69 and 0.72 respectively. They were very significantly (P<0.01) higher than those at other proportions, but AEMCP was negative. 2) Under the WCS-PV combination of different proportions, the 70∶30 ratio of WCS to PV had the largest associative effects according to the multiple factors associative effect indexes, and consequently, the 70∶30 ratio of WCS to PV was the best combination of WCS and PV (BWP); 3) Under the BWP-LC combination of different proportions, both AEGP and AEDMD were the largest at the ratio of 80∶20 and they were 0.17 and 0.18 respectively, very significantly (P<0.01) higher than those at other proportions, and the AEMCP was negative. 4) Under the BWP-LC combination of different proportions, the 80∶20 ratio of BWP to LC had the largest associative effects. In conclusion, the WCS-PV combination at the ratio of 70∶30 and WCS-PV-LC combination at the ratio of 56∶24∶20 were found to have the largest associative effects in this study.