Abstract: This study was conducted to investigate the effects of ratios of structural to nonstructural carbohydrates(SC/NSC=1.57（Ⅰ）, 1.95（Ⅱ）, 2.29（Ⅲ）)in the three diets on ruminal fermentation parameters（pH, VFA, total nitrogen, NH₃-N, UreaN，protein nitrogen）for sheep. Six Gansu high mountain fine wethers fitted with permanent ruminal cannulae, about one and half year old, and weighing 25.30 kg , were used according to a double 3×3 Latin square design with the three feeding cycles of 19 days each (10 days for adapting and 9 days for sampling). The results showed that the average of pH in rumen fluid of wethers for treatment Ⅰwere lower significantly than those for treatment Ⅱ（P<0.01） and Ⅲ（P<0.05） the pH range of all the rumen fluid were from 6.2 to 7.0 except the pH(5.86) of the treatmentⅠ was lower after 3 h of feeding in this trial. There were the tendencies that averages of TVFA concentration in rumen fluid was higher forⅠand Ⅱ than that for treatment Ⅲ（P=0.09） no significant difference on the molar ratio of acetic, propionic and other acid as well as acetic/propionic ratio among the three treatments were observed (P>0.05), the molar ratio of butyric were higher for Ⅰthan that for Ⅲ( P<0.05). As the SC/NSC being increase, the averages of total- and protein nitrogen in rumen fluid were reducing（P<0.01or P<0.05）; but no significant difference on the levels of NH₃-N and Urea-N among the three treatments occur（P>0.05）. These results indicated that as the rising of the value of SC/NSC, the pH of rumen fluid were increasing, the averages of TVFA, total and protein N concentrations were decreasing, but there were no significant influence on the levels of NH₃-N and Urea-N under portions of forage in all the three diets being higher more than 60%.