Effects of allicin on fermentation and methane emissions of in vitro Tibetan sheep with Qinghai-Ti-bet Plateau alpine meadow for four periods pastures were studied in the paper. Two healthy and age closed Tibetansheep with fistulad were used as rumen fluid donor, and in vitro technique adopted. Pastures of four periods in a year were used as substrate, and 0%, 0.5%, 1.0%, 1.5% and 2.0% of allicin were added to screen the optimum addi-tive amount. Results showed that cumulative gas production in each period decreased with the increment of allicin.When 1.5% and 2.0% of allicin were added, the decline were significant, and methane emissions decreased signifi-cantly. p H value in each group was almost consistent. When more than 1.0% allicin added in withered period and 2.0% in back green period, the NH₃-N concentration significantly declined. It had no effect on green period, butNH₃-N concentration rose significantly if 0.5% of allicin added to back green period or 1.0% of allicin added to yellow period. when above 0.5% of allicin added in withered period, or 2.0% in back green period, or 1.0% in yellowperiod, short chain fatty acid（SCFA） declined significantly. It did not changed when added in green period. Exceptof green period, organic matter digestibility（OMD） declined significantly as allicin amount reach to 2.0%. Whenabove 0.5% of allicin added in withered period, the metabolic energy（ME） decreased significantly, whereas in otherthree periods it was steady. When 2.0% of allicin added in withered period, the net energy of lactation（NEL） decreased significantly, whereas in other three periods it was steady. Therefore, when allicin was used as feed additives of Tibetan sheep, it’s optimum amount was 0.5%~1.0%.