Abstract: This experiment was conducted to study the effects of different levels of crude protein（CP） and ruminal undegraded protein （RUP） in ration on the rumen fermentation. 18 Holstein dairy cows were selected to study the effects of different levels of CP and RUP on the rumen fermentation parameters. This experiment was a 2×3 factorial arrangement with total 6 dietary combinations, the first factor was CP level （7.5%,9.0% and 10.5%）, and the second factor was RUP level （45% CP and 55% CP）. The ratio of concentration to forage was 3∶7. Pre -trial period was 10 d. The ruminal fluid samples were collected at the time points of 2, 4, 8, 12, and 24 h for 2 d. The results showed as follows： The rumen NH3-N concentration was significantly affected by CP levels of diets, but it had little effect on BCP and VFA contents. The rumen fermentation parameters for 9.0% CP level were slightly better than that of other levels; The high levels of RUP（55% CP）could reduce the pH value and NH3-N content in the rumen, and increase the concentrations of BCP and VFA in the rumen. Under the present experimental conditions, the results indicated that the treatment（9.0% CP） and RUP （55%CP） could slightly improve rumen fermentation.