Garlic contains secondary metabolites with antimicrobial properties that can alter nutrient digestibility and rumen fermentation, similar to other antimicrobial products. The objectives of the study were to evaluate the effects of garlic powder and garlic juice on in vitro nutrient digestibility, rumen fermentation, and gas production. The treatments consisted of control with no additives, garlic powder, and garlic juice at 0.5 ml and 1 ml. The digestibility of dry matter, crude protein and neutral detergent fibre were determined after 48 hours incubation. Rumen ammonia nitrogen and volatile fatty acids were determined at 12 hours and 24 hours incubation. The cumulative gas production was recorded periodically over 48 hours. The in vitro dry matter disappearance decreased with 1 ml of garlic juice compared with control. The crude protein degradability in garlic powder and garlic juice was lower than in control. Volatile fatty acids increased in all treatments. Individual volatile fatty acids were significantly different, especially propionate, whereas the acetate to propionate ratio was reduced by garlic juice, and ammonia nitrogen was reduced by garlic powder and 0.5 ml of garlic juice. The cumulative gas production increased significantly with both levels of garlic juice. The addition of garlic juice at 0.5 mL/100 ml could enhance the production of propionate, and reduce the acetate to propionate ratio, implying that the supply of hydrogen for methanogens was limited.