Four ruminally cannulated Limousin × Luxi beef cattle were used in a 4 × 4 Latin square design to monitor the changes in rumen fermentation during the feeding cycle and compare the differences caused by the type of cereal(C) and forage(F) in the diet. The 4 experimental diets contained 60% of either Leymus chinensis (LC) or sweet potato vine (SP) as forage and 40% of concentrate with either wheat (W) or maize (M). Cattle were fed twice daily and samples from rumen fluid were taken before morning feeding on the first 10 d after changing the diet and then at 0, 3, 6, 9 and 12 h after the morning feeding for 3 d. Significant variability in ruminal pH with the WSP diet was more pronounced on which pH dropped from 6.81 on d 1 (before the dietary changes) to 6.57 by d 10 (P = 0.03). The NH₃-N and total VFA concentrations of the cattle fed the Leymus chinensis were decreased (P < 0.05) after diet changes and then increased to values similar to that before changing. Whereas the total VFA concentrations of the cattle fed the sweet potato vine have the opposite trend. Mean rumen pH, acetate and propionate proportion and the ratio of acetate to propionate were not affected by the treatments (P > 0.05). The cattle fed WSP diet had greater (P < 0.05) NH₃-N and total VFA concentrations and greater (P < 0.05) proportion of butyrate. The average pH values and the ratio of acetate to propionate were affected by cereal (P = 0.004, P = 0.046 respectively) and the concentrations of total VFA, the proportion of propionate and butyrate were affected by forage (P = 0.006, P = 0.045, P < 0.001, respectively). No cereal × forage interactions were detected among all rumen fermentation characteristics (P > 0.05). The rumen pH values and concentrations of NH₃-N and total VFA were fluctuated at certain sampling days after the abrupt change of diets, however, at the end of the adaptation period the rumen fermentation characteristics were similar to the values before diets changes. The result also indicated that the type of cereal and forge had apparent effects on rumen fermentation characteristics, but no interactions were observed.