Abstract: This study was conducted to investigate the effects of diet pellets with different concentrate-roughage ratios on rumen microbial protein synthesis, fermentation and microorganism abundance in weaned bull calves. A total of 12 healthy, weaned Holstein bull calves (age=103.92±1.5 d; weight=121.25±4.12 kg) were randomly assigned to 4 groups (Ⅰ, Ⅱ, Ⅲ and Ⅳ), with 3 calves in each group. The treatment diets were complete-diet pellets with four concentrate-roughage ratios (75∶25, Ⅰ; 70∶30, Ⅱ; 65∶35, Ⅲ; 60∶40, Ⅳ). The experiment ran for 70 days in total, including 14 d for adaptation and 56 d for the trial itself. Rumen microbial protein synthesis, fermentation and microbial abundance were measured. The results showed that total volatile fatty acids (VFA) and acetate concentration in group Ⅳ were the lowest, and were significantly lower than those in groupⅠ(P<0.05). The butyrate level of group Ⅳ was significantly higher than groupsⅠand Ⅲ (P<0.05). Group Ⅲ had the highest A∶P, which was significantly greater than group Ⅳ(P<0.05). The relative abundance of Ruminococcus albus, Butyrivibrio fibrisolvens and general fungi in group Ⅳ was significantly higher than the other three groups (P<0.05). Ruminococcus flavefaciens abundance was the highest in group Ⅲ when compared with the other groups (P<0.05). The results indicate that rumen total VFA and acetate concentration can be increased in high-concentrate groups, while it will inhibit growth of the fibre-adherent rumen bacteria and anaerobic fungi.