The trial was conducted to study the effects of different proportions of fresh alfalfa on rumen digestion and metabolism of Boer goats. Four male Boer goats with an average body weight of (39.8±1.45) kg with permanent cannulas were used in a 4×4 Latin square design. The four treatments were consisted of the same concentrates but with different roughages. The four kinds of roughages were composed of 100% chopped peanut vine (control group), 85% chopped peanut vine+15% fresh alfalfa (group Ⅰ), 70% chopped peanut vine+30% fresh alfalfa (group Ⅱ), and 55% chopped peanut vine+45% fresh alfalfa (group Ⅲ), respectively. The experimental period lasted for 60 days. The results showed as follows: 1) degradability of DM and CP increased gradually with the proportion of fresh alfalfa increased, and their degradability in group Ⅲ were significantly higher than those in control group (P<0.05); 2) degradability of NDF and ADF showed a tendency of increased at first and then decreased with the proportion of fresh alfalfa increased, and degradability of NDF and ADF in group Ⅱ were significantly higher than those in control group (P<0.05); 3) the acetic acid content and butyric acid content increased at first and then decreased with the proportion of fresh alfalfa increased except at 10 h of incubation, and the propionic acid content had the same tendency at 8 and 9 h of incubation; 4) NH3-N content increased gradually with the proportion of fresh alfalfa increased except at 9 h of incubation, and that of group Ⅲ was the highest; 5) there was no significant difference in rumen pH among each group (P>0.05). In conclusion, substitution of fresh alfalfa for chopped peanut vine in the diets of Boer goats can increase the digestibility of DM, CP, NDF and ADF of roughage, and increase the volatile fatty acid contents, especially the acetic acid as well as NH3-N contents. The optimal proportion of fresh alfalfa was 30% in this trial.［Chinese Journal of Animal Nutrition，2011，23（1）：162-170］