This study was conducted to evaluate effects of normal and deodorized garlic oil on rumen fermentation, methanogenesis and microbiota in vitro. Reading pressure technique system and gas chromatography were used to determine gas production and methane emission, respectively. Volatile fatty acids, pH and ammonia nitrogen were determined after 24 h incubation. Population of rumen microbiota was detected by real-time PCR assay. The results showed that addition of garlic oil significantly decreased methane production (P<0.01), acetate proportion and ammonia nitrogen (P<0.05), and increased propionate proportion (P<0.05). The gas production and total VFA concentration were influenced but the effect was less than that on the methane production. Addition of deodorized garlic oil tended to influence methane emission and gas production (P>0.05). Whether adding normal or deodorized garlic oil, proportion of protozoa and methanogen related to total rumen bacterial 16S rDNA was decreased significantly (P<0.01), with a stronger effect by garlic oil than that by deodorized one (P<0.01). In conclusion, addition of garlic oil at a proper level could change the fermentation pattern, increase the proportion of propionate to acetate, and inhibit methanogenesis. These were associated with the change of microbiota in the rumen. Normal garlic oil had a stronger effect than the deodorized one.［Chinese Journal of Animal Nutrition, 2010，22（2）:386-392］