The effect of tea saponins (TS) on rumen fermentation and methane emission was examined using an in vitro gas production technique named Reading Pressure Technique. Three levels of TS addition (0, 0.2, 0.4 mg/ml) were evaluated in the faunated and defaunated rumen fluid. Compared to the control, TS addition decreased the 24 h gas production in the faunated rumen fluid, but had a minor effect on gas yield in the defaunated rumen fluid. The TS significantly reduced methane production in vitro. In the faunated rumen fluid, 0.2 or 0.4 mg/ml TS decreased the 24 h methane emission by 12.7% or 14.0%, respectively.Rumen fluid pH value was affected neither by TS addition nor by defaunation. The TS addition had only minor effects on volatile fatty acids, but the yield and pattern of volatile fatty acids were greatly affected by defaunation. While the molar proportion of acetate was not affected by defaunation, the propionate was significantly increased and the butyrate significantly decreased.Ammonia-N concentration and microbial protein yield were influenced by TS inclusion and defaunation. Inclusion of 0.4 mg/ml TS increased the microbial protein mass by 18.4% and 13.8% and decreased the ammonia-N concentration by 8.3% and 19.6% in the faunated and defaunated rumen fluid, respectively. Protozoa counts were significantly reduced by TS inclusion. The current study demonstrated the beneficial effect of TS on methane production and rumen fermentation, and indicated that this may be due to the effect of the associated depression on protozoa counts.