The present study evaluated the effects of different doses and different adaptation time of ginger powder on rumen fermentation in vivo. In Experiment 1, four sheep were used in a 4 x 4 Latin square design to evaluate the effects of three concentrations (5, 10, and 20 g/kg) of ginger powder on rumen fermentation. Total volatile fatty acid (VFA) concentration was decreased (P<0.05), but the fatty acid compositions, acetate-to-propionate ratio, pH, and ammonia-N concentration were unchanged by the addition of ginger powder. In Experiment 2, eight sheep were used to study the long-term adaptation of ginger powder on rumen fermentation. The addition of ginger powder decreased (P<0.05) the total VFA concentration from days 5 to 20, but these differences disappeared thereafter. Other rumen fermentation parameters were unchanged when ginger powder was added. Therefore, data from short-term in vitro fermentation studies may lead to erroneous conclusions and should be interpreted with caution.